refactor(mwdt): move sleep retention config into system layer

Move MWDT retention descriptors out of esp_hal_wdt and into esp_system
so the backup policy lives with the watchdog users, while keeping the
per-chip retention sources under the existing port/soc target
directories.

components/esp_system/port/include/private/mwdt_priv.h

@@ -0,0 +1,31 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include "soc/soc_caps.h"
+#include "hal/mwdt_ll.h"
+#if SOC_PAU_SUPPORTED
+#include "soc/regdma.h"
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if SOC_PAU_SUPPORTED
+typedef struct {
+    const regdma_entries_config_t *regdma_entry_array;
+    uint32_t array_size;
+} mwdt_reg_retention_info_t;
+
+extern const mwdt_reg_retention_info_t mwdt_reg_retention_info[TIMG_LL_GET(INST_NUM)];
+#endif
+
+#ifdef __cplusplus
+}
+#endif