rtc: ab-eoz9: set regmap max_register

Set regmap's max_register property to allow users to dump registers
using debufgs.

Signed-off-by: Liam Beguin <lvb@xiphos.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210408024028.3526564-2-liambeguin@gmail.com