ARM: dts: at91: Fix typo in ISC_D0 on PC9

The function argument for the ISC_D0 on PC9 was incorrect. According to
the documentation it should be 'C' aka 3.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Fixes: 7f16cb67 ("ARM: at91/dt: add sama5d2 pinmux")
Cc: <stable@vger.kernel.org> # v4.4+