mmc: omap: follow TRM procedure to power on cards

According to OMAP3 TRM, PBIASLITEPWRDNZ bits must be cleared while MMC
power supply is being enabled and is ramping up (those bits might be
left set by the previous bootloader). It doesn't say what happens if
this procedure is violated, but better not to risk here and do things
as required.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Tom Rini <trini@ti.com>