char/genrtc: remove m68k support

The asm/rtc.h header is only used for the old gen_rtc driver
that has been replaced by rtc-generic. According to Geert
Uytterhoeven, nobody has used the old driver on m68k for
a long time, so we can now just remove the header file
and disallow the driver in Kconfig.

All files that used to include asm/rtc.h are now changed so
they include the headers that were used implicitly through
asm/rtc.h.

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