[U-Boot] disabling mmc in spl when booting using bootrom

[Sandy Patterson]

>
> My concern with the Rockchip back-to-brom feature is that then U-Boot
> cannot load U-Boot normally as SPL likes to do. I think it is fine
> when we are short on space, but OF_PLATDATA fixes that.
>
> So I think the back-to-rom feature need to be optional on boards which
> don't have to use it, since it uses a non-standard boot approach,
>
>
It seems like OF_PLATDATA could be used across the board for RK3288. We
found that the amount of code changes in the two approaches made us more
comfortable using the RK3288 BOOTROM to load u-boot proper. But that was
for our specific use case.

Thanks for the feedback. I'll abandon this changeset. Kever is essentially
doing what you recommend in his SDRAM patches anyway.

Sandy