MAINTAINERS: add path for UART samples
MAINTAINERS: add path for UART samples

Adding UART samples to "Drivers: Serial/UART" so that the maintainer
will be notified for relevant PRs in the future.

Signed-off-by: Martin Jäger <54669547a225ff20cba8b75a4adca540eef25858@libre.solar>

galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr