ARM: dts: at91: move mmc pinctrl-names property to board dts

Having the pinctrl-names property in the dtsi leads to dtbs_check warnings
when the board dts doesn't define pinctrl-0. Instead, move the property to
the board dts actually using the mmc node.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20200831171129.3886857-5-alexandre.belloni@bootlin.com