armv7/ls1021atwr: program the regulator for deep sleep

Program the external regulator to switch off voltage in deep sleep.

Signed-off-by: Chenhui Zhao <chenhui.zhao@freescale.com>
Acked-by: Alison Wang <alison.wang@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>