char/genrtc: x86: remove remnants of asm/rtc.h

Commit 3195ef59 ("x86: Do full rtc synchronization with ntp") had
the side-effect of unconditionally enabling the RTC_LIB symbol on x86,
which in turn disables the selection of the CONFIG_RTC and
CONFIG_GEN_RTC drivers that contain a two older implementations of
the CONFIG_RTC_DRV_CMOS driver.

This removes x86 from the list for genrtc, and changes all references
to the asm/rtc.h header to instead point to the interfaces
from linux/mc146818rtc.h.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>