boards: arduino_zero: Fix SPI pinmux configuration.

This commit changes the Arduino Zero pinmux configuration to use PA12
as MISO.

According to the schematic, the Arduino Zero provides a SPI bus on
SERCOM4 with the following pads/pins:
 - MISO: PA12/pad0 (pin 21)
 - MOSI: PB10/pad2 (pin 19)
 - SCK:  PB11/pad3 (pin 20)

The MISO signal is incorrectly labeled as PB12_S4_SPI_MISO on the
schematic. It should be labeled PA12_S4_SPI_MISO.

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>