drivers: uart_gecko: use DT_<COMPAT>_<INSTANCE>_<PROP> defines

Use the new DT_<COMPAT>_<INSTANCE>_<PROP> defines to instantiate
devices. This commit adds also ability to define individual pin
locations on SoC series that support the feature. Definitions of GPIO
pins assigned to a given location have been moved from soc_pinmap.h file
to board DTS file.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>