x86: unify mc146818rtc.h - prepare for sharing rtc code

Unify mc146818rtc.h by adding the rtc_cmos_read/write functions to
time_64.c. This is a preparatory patch to finaly share the rtc code,
which is unsurprisingly similar.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>