drivers: mmc: omap_hsmmc: Fix IO Buffer on OMAP36xx

On the OMAP36xx/37xx the CONTROL_WKUP_CTRL register has
a field (bit 6) named GPIO_IO_PWRDNZ.  If 0, the IO buffers which
are related to GPIO_126, 127 and 129 are disabled. Some boards may
need this for MMC. After the PBIAS is configured, this bit should
be set high to enable these GPIO pins.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>