feat(esp8266): Add reset reason function

The function can be disable.

components/esp8266/include/esp8266/rtc_register.h

@@ -0,0 +1,29 @@
+// Copyright 2018-2019 Espressif Systems (Shanghai) PTE LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#pragma once
+
+typedef enum {
+    NO_MEAN                =  0,
+    POWERON_RESET          =  1,    /**<1, Vbat power on reset*/
+    EXT_RESET              =  2,    /**<2, external system reset*/
+    SW_RESET               =  3,    /**<3, Software reset digital core*/
+    OWDT_RESET             =  4,    /**<4, Legacy watch dog reset digital core*/
+    DEEPSLEEP_RESET        =  5,    /**<5, Deep Sleep reset digital core*/
+    SDIO_RESET             =  6,    /**<6, Reset by SLC module, reset digital core*/
+} RESET_REASON;
+
+#define RTC_STORE0                  (REG_RTC_BASE + 0x30)
+#define RTC_STATE1                  (REG_RTC_BASE + 0x14)
+#define RTC_STATE2                  (REG_RTC_BASE + 0x18)