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/soc/esp32c6/include/soc/Kconfig.soc_caps.in

@@ -771,6 +771,10 @@ config SOC_TIMER_GROUP_SUPPORT_PLL_F80M
     bool
     default y
 
+config SOC_TIMER_GROUP_SUPPORT_RC_FAST
+    bool
+    default y
+
 config SOC_TIMER_GROUP_TOTAL_TIMERS
     int
     default 2

components/soc/esp32c6/include/soc/clk_tree_defs.h

@@ -149,15 +149,16 @@ typedef enum {
  * }
  * @endcode
  */
-#define SOC_GPTIMER_CLKS {SOC_MOD_CLK_PLL_F80M, SOC_MOD_CLK_XTAL}
+#define SOC_GPTIMER_CLKS {SOC_MOD_CLK_PLL_F80M, SOC_MOD_CLK_RC_FAST, SOC_MOD_CLK_XTAL}
 
 /**
  * @brief Type of GPTimer clock source
  */
 typedef enum {
-    GPTIMER_CLK_SRC_PLL_F80M = SOC_MOD_CLK_PLL_F80M,   /*!< Select PLL_F80M as the source clock */
-    GPTIMER_CLK_SRC_XTAL = SOC_MOD_CLK_XTAL,           /*!< Select XTAL as the source clock */
-    GPTIMER_CLK_SRC_DEFAULT = SOC_MOD_CLK_PLL_F80M,    /*!< Select PLL_F80M as the default choice */
+    GPTIMER_CLK_SRC_PLL_F80M = SOC_MOD_CLK_PLL_F80M, /*!< Select PLL_F80M as the source clock */
+    GPTIMER_CLK_SRC_RC_FAST = SOC_MOD_CLK_RC_FAST,   /*!< Select RC_FAST as the source clock */
+    GPTIMER_CLK_SRC_XTAL = SOC_MOD_CLK_XTAL,         /*!< Select XTAL as the source clock */
+    GPTIMER_CLK_SRC_DEFAULT = SOC_MOD_CLK_PLL_F80M,  /*!< Select PLL_F80M as the default choice */
 } soc_periph_gptimer_clk_src_t;
 
 /**

components/soc/esp32c6/include/soc/soc_caps.h

@@ -359,6 +359,7 @@
 #define SOC_TIMER_GROUP_COUNTER_BIT_WIDTH (54)
 #define SOC_TIMER_GROUP_SUPPORT_XTAL      (1)
 #define SOC_TIMER_GROUP_SUPPORT_PLL_F80M  (1)
+#define SOC_TIMER_GROUP_SUPPORT_RC_FAST   (1)
 #define SOC_TIMER_GROUP_TOTAL_TIMERS      (2)
 #define SOC_TIMER_SUPPORT_ETM             (1)