[U-Boot] USB EHCI support

[Aaron Williams]

I'm working on adding EHCI support for our chips to U-Boot and I'm a bit 
confused. In our case I have to replace the ehci_read/writel macros since all 
of our EHCI registers are located starting at 0x800016F0000000400 (notice it's 
a 64-bit address).

Anyway, to get back to my comment, it seems that CONFIG_EHCI_DESC_BIG_ENDIAN 
and CONFIG_EHCI_MMIO_BIG_ENDIAN are swapped. In our case, MMIO to our 
registers is always big endian but we can configure the buffer and descriptor 
parts to be big or little endian (and several other combinations).

-Aaron