ARM: dts: at91: sam9x60: fix mux-mask for PA7 so it can be set to A, B and C

According to the datasheet PA7 can be set to either function A, B or
C (see table 6-2 of DS60001579D). The previous value would permit just
configuring with function C.

Signed-off-by: Federico Pellegrin <fede@evolware.org>
Fixes: 1e5f532c ("ARM: dts: at91: sam9x60: add device tree for soc and board")
Cc: <stable@vger.kernel.org> # 5.6+
Cc: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>