rtc: ab3100: set range

The ab3100 has a 48bit counter running at 65536 Hz (despite one of the
comment). The max value is then (2^48 - 1)/2^16 == 2^32 - 1.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>