mmc: Add mmc_vddrange_to_ocrmask() helper function

This function sets the OCR mask bits according to provided voltage
ranges. Will be used by the mmc_spi OpenFirmware bindings.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>