ARM: dts: at91: sama7: Align with Linux Devicetree

This patch makes sure that the Devicetree for the sama7
boards are aligned with the Devicetree from Linux. This
implies removing the GPIO compatible and replacing it
with the PINCTRL one, as well as unifying the SDMMC
pinctrl related subnodes under one single subnode.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>