rtc: rtc-sh: Support 4-digit year on SH7705/SH7710/SH7712.

All SH-4 parts have a 4-digit year, while the SH-3 parts typically
only use a 2-digit one. The SH7705, SH7710, and SH7712 SH-3 parts
however opted to extend it to 4-digit and still look and act like
an SH-3 RTC in all other ways.

This adds a capability flag (RTC_CAP_4_DIGIT_YEAR) that these
corner-case CPU subtypes can set in their platform data and cleans
up some of the ifdef mess in the driver as a result.

Reported-by: Markus Brunner <super.firetwister@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>