clocksource/drivers/Kconfig: Fix CLKSRC_PISTACHIO dependencies

In v4.13, CLKSRC_PISTACHIO can select TIMER_OF on architectures without
GENERIC_CLOCKEVENTS, resulting in a struct clock_event_device missing
some required features and build breakage compiling timer_of.c. One of
the symbols selecting TIMER_OF is CLKSRC_PISTACHIO, so add the
dependency on GENERIC_CLOCKEVENTS.

Thanks to kbuild test robot for finding this error
(https://lkml.org/lkml/2017/7/16/249

)

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Suggested-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>