drivers: serial: uart_sam0: Fix sercom5 pinctrl pad per datasheet

Fix a typo bug where sercom5 pinctrl has pad2 and pad3
has duplicated pads for pb0/pb2 and pb1/pb3.
Per the datasheet, pb2 and pb3 should be pad0 and pad1 respectively.

Signed-off-by: Ron Smith <rockyowl171@gmail.com>