ARM: multi_v7_defconfig: Enable PBIAS regulator

PBIAS regulator is required for MMC module in OMAP2, OMAP3, OMAP4,
OMAP5 and DRA7 SoCs. Enable it here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>