drivers/clocksource/pistachio: Fix memory corruption in init

Driver init code incorrectly uses the block base address and as a result
clears clocksource structure's fields instead of the hardware registers.

Commit 09a99820 ("timekeeping: Lift clocksource cacheline
restriction") has changed the offsets within pistachio_clocksource
structure and what has previously gone unnoticed now leads to a kernel
panic during boot.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>