libpayload/drivers/nvram: Add function to write RTC

Add a function to set the RTC to provided struct tm.

Change-Id: I17b4c1ee0dcc649738ac6a7400b087d07213eaf0
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/23585
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/include/libpayload.h

@@ -151,6 +151,7 @@ u8 nvram_read(u8 addr);
 void nvram_write(u8 val, u8 addr);
 int nvram_updating(void);
 void rtc_read_clock(struct tm *tm);
+void rtc_write_clock(const struct tm *tm);
 /** @} */
 
 /**