drivers: serial: Microchip MEC5 add select PINCTRL to Kconfig

Zephyr has changed the rules for PINCTRL to use Kconfig select
feature at the driver level instead of setting the Kconfig
item directly in the board or application level. We updated
the MEC5 UART serial driver Kconfig to select PINCTRL.

Signed-off-by: Scott Worley <scott.worley@microchip.com>