[U-Boot] mmc: use core clock frequency in bcm2835 sdhost

[Alexander Graf]

> In raspberrypi-firmware 7fdcd00e00a42a1c91e8bd6f5eb8352fe9358557 and
> later start.elf now sets the EMMC clock to 200 MHz.
> 
> According to Phil Elwell in
> https://github.com/raspberrypi/firmware/issues/953
> the SDHost controller shares the core/VPU clock and doesn't use
> the EMMC clock.
> 
> Use the core clock id when determining the frequency to allow
> U-Boot to work with recent versions of raspberrypi-firmware.
> Otherwise U-Boot hangs at:
> 
> U-Boot 2018.03 (Mar 14 2018 - 20:36:00 +1100)
> 
> DRAM:  948 MiB
> RPI 3 Model B (0xa02082)
> MMC:   mmc at 7e202000: 0, sdhci at 7e300000: 1
> Loading Environment from FAT...
> 
> Signed-off-by: Jonathan Gray <jsg at jsg.id.au>
> Cc: Alexander Graf <agraf at suse.de>
> Cc: Peter Robinson <pbrobinson at gmail.com>

Thanks, applied to rpi-next

Alex