[nrf fromlist] 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.

Upstream PR #: 87807

Signed-off-by: Krzysztof Chruściński <[email protected]>
(cherry picked from commit 2bd58c7)

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