char/genrtc: remove parisc support

This architecture selects RTC_CLASS unconditionally, so the GEN_RTC
has not worked here for a long time.

Now we can remove both the asm/rtc.h header and the Kconfig dependency
for CONFIG_GEN_RTC.

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>