mmc: Merge SD_LEGACY and MMC_LEGACY bus modes

MMC_LEGACY & SD_LEGACY are not differentiated timings in the spec and
don't have any meaningful differences. Therefore, get rid of all
references to SD_LEGACY and use MMC_LEGACY to mean both of them.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>