Enable driver model support for MMC and SATA.

Note that DM_MMC requires aliases for your mmc devices so
they are added to the dts. Linux does not support enumerating mmc
devices by alias so these are not present in the Linux dts.

Note that we still need board_mmc_init() and board_mmc_getcd() for
not DM SPL to support MMC.
Signed-off-by: NTim Harvey <tharvey@gateworks.com>