char/genrtc: remove asm-generic/rtc.h from mips

arch/mips/sni/time.c includes asm-generic/rtc.h for no apparent reason,
and it works fine without that header, so lets remove the inclusion
in preparation of deleting the file.

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>