rtc: rx8010: Specify correct address for RX8010_RESV31

Define for reserved register 31 had the incorrect address. Specify
the correct address.

Reported-by: Jens-Peter Oswald <oswald@lre.de>
Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>