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/include/esp_psram.h

@@ -42,6 +42,24 @@ bool esp_psram_is_initialized(void);
  */
 size_t esp_psram_get_size(void);
 
+/**
+ * @brief Check if the pointer falls inside the unencrypted PSRAM carve-out region
+ *
+ * When @c CONFIG_SPIRAM_ENC_EXEMPT is enabled, esp_psram reserves a region of PSRAM
+ * that is mapped without encryption and exposed through the @c MALLOC_CAP_SPIRAM_NO_ENC
+ * heap capability. This function lets drivers verify whether a buffer returned by the
+ * heap allocator actually lives in that unencrypted region — useful for example after
+ * a @c heap_caps_malloc_prefer() call that may have fallen back to encrypted PSRAM.
+ *
+ * @param[in] p  The pointer to check
+ *
+ * @return
+ *        - true:  the pointer is within the unencrypted PSRAM carve-out
+ *        - false: the pointer is not in the carve-out, PSRAM is not initialized,
+ *                 or @c CONFIG_SPIRAM_ENC_EXEMPT is disabled
+ */
+bool esp_psram_ptr_is_no_enc(const void *p);
+
 #ifdef __cplusplus
 }
 #endif