ARM: dts: imx6qdl-sabresd: Set LDO regulator supply

Setting the supply is optional but beneficial, it will cause PMIC
voltages to be dynamically changed with cpu frequency.

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>