reboot-mode: read the boot mode from RTC memory

RTC devices could provide battery-backed memory that can be used for
storing the reboot mode magic value.

Add a new reboot-mode back-end that uses RTC to store the reboot-mode
magic value. The driver also supports both endianness modes.

Signed-off-by: Nandor Han <nandor.han@vaisala.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/reboot-mode/reboot-mode-rtc.h

@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c), Vaisala Oyj
+ */
+
+#ifndef REBOOT_MODE_REBOOT_MODE_RTC_H_
+#define REBOOT_MODE_REBOOT_MODE_RTC_H_
+
+struct reboot_mode_rtc_platdata {
+	struct udevice *rtc;
+	bool is_big_endian;
+	int addr;
+	size_t size;
+};
+
+#endif /* REBOOT_MODE_REBOOT_MODE_RTC_H_ */