esp_hw_support: add 26 MHz XTAL option for esp32c2

Some esp32c2 boards will be produced with a 26 MHz XTAL. This commit
adds the basic Kconfig option for this type of hardware.
Support for CONFIG_ESP32C2_XTAL_FREQ_26 in other areas of IDF will be
implemented in subsequent commits.

components/esp_hw_support/port/esp32c2/Kconfig.hw_support

@@ -0,0 +1,21 @@
+config ESP32C2_XTAL_FREQ
+    int
+    default 40 if ESP32C2_XTAL_FREQ_40
+    default 26 if ESP32C2_XTAL_FREQ_26
+
+choice ESP32C2_XTAL_FREQ_SEL
+    prompt "Main XTAL frequency"
+    default ESP32C2_XTAL_FREQ_40
+    help
+        ESP32-C2 currently supports the following XTAL frequencies:
+
+        - 26 MHz
+        - 40 MHz
+
+        This option must be set to the correct value for the given hardware.
+
+    config ESP32C2_XTAL_FREQ_40    # TODO: IDF-5488
+        bool "40 MHz"
+    config ESP32C2_XTAL_FREQ_26
+        bool "26 MHz"
+endchoice