ARM: dts: imx6sx-sdb: Properly model the ARM and SOC regulator supplies

On imx6sx-sdb rev B/C the VDD_ARM_IN and VDD_SOC_IN supplies are connected
together and both are supplied by the SW1A PMIC output, so model this
correctly in the device tree.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>