[U-Boot] u-boot device model SPI + SPI Flash

[Hoefle Marco]

 Hello,
I saw that here is also a mailing list dedicated to device model issues but it looks obsolete (last post February 2014) so I am using this forum.

I hav​e a problem nearly identically to
http://lists.denx.de/pipermail/u-boot/2015-April/210754.html


The SPI initialization does not work:​ 

Invalid bus 0 (err=-19)
*** Warning - spi_flash_probe() failed, using default environment


This is the device tree compiled into u-boot:

U-Boot> fdt addr 0x8FEB8A7C
U-Boot> fdt print
/ {
        #address-cells = <0x00000001>;
        #size-cells = <0x00000001>;
        aliases {
                serial0 = "serial at 40600000";
                spi0 = "/axi_quad_spi at 44a00000";
        };
        memory {
                device_type = "memory";
                reg = <0x80000000 0x10000000>;
        };
        axi_quad_spi at 44a00000 {
                compatible = "xlnx,xps-spi-2.00.a";
                reg = <0x44a00000 0x00010000>;
                spi-max-frequency = <0x423a35c7>;
                #address-cells = <0x00000001>;
                #size-cells = <0x00000000>;
                status = "okay";
                is-dual = <0x00000000>;
                num-cs = <0x00000001>;
                flash at 0 {
                        compatible = "n25q128a11";
                        reg = <0x00000000>;
                        spi-tx-bus-width = <0x00000001>;
                        spi-rx-bus-width = <0x00000004>;
                        spi-max-frequency = <0x0d3ed78e>;
                        #address-cells = <0x00000001>;
                        #size-cells = <0x00000001>;
                        partition at qspi-fsbl-uboot {
                                label = "qspi-fsbl-uboot";
                                reg = <0x00000000 0x00100000>;
                        };
                        partition at qspi-linux {
                                label = "qspi-linux";
                                reg = <0x00100000 0x00500000>;
                        };
                        partition at qspi-device-tree {
                                label = "qspi-device-tree";
                                reg = <0x00600000 0x00020000>;
                        };
                        partition at qspi-rootfs {
                                label = "qspi-rootfs";
                                reg = <0x00620000 0x005e0000>;
                        };
                        partition at qspi-bitstream {
                                label = "qspi-bitstream";
                                reg = <0x00c00000 0x00400000>;
                        };
                };
        };
};

Has anybody an idea how to debug this further?
Thanks,
Marco​