mmc: sdhci: add hooks for setting UHS in platform specific code

Allow platform specific code to set UHS registers if
implementation requires speciial platform specific handling

Signed-off-by: Philip Rakity <prakity@marvell.com>
Reviewed-by: Arindam Nath <arindam.nath@amd.com>
Signed-off-by: Chris Ball <cjb@laptop.org>