rtc: ls1x: add range

While the year is encoded on 32 bits in SYS_TOYWRITE1i/SYS_TOYREAD1. The
Loongson 1c datasheet states that the range is from 0 to 99.

The current code exceeds this range and seems to be working, I deduce that
the leap year algorithm will fail in 2100.

Anyway, alarm registers only encode the year on 14 bits so with alarm
support, the range will always be limited to 0 to 16383.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>