char/genrtc: remove the rest of the driver

No architecture uses the genrtc driver any more, so let's kill it off
for good. This now also includes asm-generic/rtc.h, which is otherwise
completely unused.

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>