tests: drivers: uart: uart_pm: Disable device runtime PM

Explicitly disable device runtime PM since this test is expecting
it to be disabled and there are SoC now that turns on it by default.

Signed-off-by: Krzysztof Chruściński <[email protected]>

Author: nordic-krch

tests/drivers/uart/uart_pm/prj.conf

@@ -2,3 +2,4 @@ CONFIG_ZTEST=y
 CONFIG_SERIAL=y
 CONFIG_PM=y
 CONFIG_PM_DEVICE=y
+CONFIG_PM_DEVICE_RUNTIME=n