rtc: cmos: don't refer to asm-generic/rtc.h

That header has been gone for a while.  I've fixed up the Kconfig
comment, but the one in rtc-cmos.c doesn't make any sense to me
even looking at its history.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>