regulator: pbias: Handle extended drain IO when changing omap36 PBIAS

The OMAP36 and DM37 TRM state to disable extneded drain IO before
changing the PBIAS.  This patch does this before pmic writes if
the CONFIG_MMC_OMAP36XX_PINS flag is set and the cpu family is
omap36xx

Signed-off-by: Adam Ford <aford173@gmail.com>