drivers: serial: uart_sam0: Add pinctrl support

Add Atmel sam0 sercom[uart] pinctrl bindings and implements pinctrl at
driver level.  It changes all sam0 boards to use new feature and remove
pinmux driver dependency for sercom[uart].  The samples that require a
binding were update to keep consistency and avoid errors.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>