feat(esp_psram): add esp_psram_ptr_is_no_enc() helper

Drivers that allocate from the unencrypted PSRAM carve-out via
MALLOC_CAP_SPIRAM_NO_ENC currently have no way to verify after the fact
which pool a buffer came from. This is particularly relevant for callers
using heap_caps_malloc_prefer(MALLOC_CAP_SPIRAM_NO_ENC, MALLOC_CAP_SPIRAM),
where a silent fallback to encrypted PSRAM would still pass the typical
esp_ptr_external_ram() check.

Expose esp_psram_ptr_is_no_enc() in the public esp_psram.h header. It
performs a range check against the carve-out's virtual-address window and
returns false when PSRAM is not initialized or CONFIG_SPIRAM_ENC_EXEMPT is
disabled, so callers do not need to guard the call site with #if.

Also reference the helper from the External RAM documentation alongside
the heap_caps_malloc(MALLOC_CAP_SPIRAM_NO_ENC) usage example.

components/esp_psram/system_layer/esp_psram.c

@@ -618,6 +618,21 @@ bool IRAM_ATTR esp_psram_check_ptr_addr(const void *p)
     return false;
 }
 
+bool IRAM_ATTR esp_psram_ptr_is_no_enc(const void *p)
+{
+#if CONFIG_SPIRAM_ENC_EXEMPT
+    if (!s_psram_ctx.is_initialised) {
+        return false;
+    }
+
+    return ((intptr_t)p >= s_psram_ctx.mapped_regions[PSRAM_MEM_ENC_EXEMPT].vaddr_start &&
+            (intptr_t)p < s_psram_ctx.mapped_regions[PSRAM_MEM_ENC_EXEMPT].vaddr_end);
+#else
+    (void)p;
+    return false;
+#endif
+}
+
 esp_err_t esp_psram_extram_reserve_dma_pool(size_t size)
 {
     if (size == 0) {