drivers/rtc/rtc-isl12057.c: add support for century bit

The month register of ISL12057 RTC chip includes a century bit which
reports overflow of year register from 99 to 0.  This bit can also be
written, which allows using it to extend the time interval the chip can
support from 99 to 199 years.

This patch adds support for century overflow bit in tm to regs and regs to
tm helpers in ISL12057 driver.

This was tested by putting a device 100 years in the future (using a
specific kernel due to the inability of userland tools such as date or
hwclock to pass year 2038), rebooting on a kernel w/ this patch applied
and verifying the device was still 100 years in the future.

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Suggested-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Acked-by: Uwe Kleine-König <uwe@kleine-koenig.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Peter Huewe <peter.huewe@infineon.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>