pinctrl: gecko: fix compilation and UART handling

LEUART_Typedef isn't defined for every possible target.
It should be included in the conditional compilation part.

For proper handling of UART location, the driver needs
to remember pin configuration of both TX and RX.
This was broken in #60695 is brought back here.

Signed-off-by: Wojciech Sipak <wsipak@antmicro.com>