gptimer: support RC_FAST clock source

GPTimer on C6 supports RC_FAST as the clock source. The benefit of this
clock is that, the clock can sustain in light sleep.

Added a new API to return the real resolution of the timer handle.

components/driver/test_apps/gptimer/pytest_gptimer.py

@@ -6,7 +6,6 @@ from pytest_embedded import Dut
 
 
 @pytest.mark.supported_targets
-@pytest.mark.temp_skip_ci(targets=['esp32c6'], reason='iram_safe test failed')
 @pytest.mark.generic
 @pytest.mark.parametrize(
     'config',