ARM: dts: imx6qp-sabresd: Set reg_arm regulator supply

On imx6qp-sabresd LDO_ARM is connected to a different PMIC output than
the other imx6qdl-sabresd boards.

Setting cpu0 arm-supply to sw2_reg is wrong, this must have mistakenly
slipped out of the vendor tree where this is are used for LDO bypass.

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>