rtc: rx8581: Add support for Epson rx8571 RTC

Add support for Epson rx8571 real-time clock. rx8571 rtc is compatible
with rx8581,except that rx8571 has additional 16 bytes of RAM.

16 bytes of nvmem is supported and exposed in sysfs (# is the instance
number,starting with 0): /sys/bus/nvmem/devices/rx8571-#/nvmem

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>