The register range for the RTC extends beyond 0x54.
Use the size from the user manual's memory map instead.

Fixes: 9765d2d9 ("rtc: sun6i: Add sun6i RTC driver")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>