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/hal/esp32c6/include/hal/timer_ll.h

@@ -68,6 +68,9 @@ static inline void timer_ll_set_clock_source(timg_dev_t *hw, uint32_t timer_num,
     case GPTIMER_CLK_SRC_PLL_F80M:
         clk_id = 1;
         break;
+    case GPTIMER_CLK_SRC_RC_FAST:
+        clk_id = 2;
+        break;
     default:
         HAL_ASSERT(false);
         break;