heap: add rtc fast memory region to dynamic pool

- for ESP32 only enabled in case of unicore config
- capability wise this region (8K) is same as DRAM, except non-DMA capable
- also fixed small issue in reserved memory region processing when (start == end)

components/esp32s2/Kconfig

@@ -498,6 +498,16 @@ menu "ESP32S2-specific"
             If enabled, this disables the linking of binary libraries in the application build. Note
             that after enabling this Wi-Fi/Bluetooth will not work.
 
+    config ESP32S2_ALLOW_RTC_FAST_MEM_AS_HEAP
+        bool "Enable RTC fast memory for dynamic allocations"
+        depends on !ESP32S2_MEMPROT_FEATURE
+        default y
+        help
+            This config option allows to add RTC fast memory region to system heap with capability
+            similar to that of DRAM region but without DMA. This memory will be consumed first per
+            heap initialization order by early startup services and scheduler related code. Speed
+            wise RTC fast memory operates on APB clock and hence does not have much performance impact.
+
 endmenu  # ESP32S2-Specific
 
 menu "Power Management"