clocksource/drivers/sp804: Prepare for support non-standard register offset

Add two local variables: timer1_base and timer2_base in sp804_of_init(),
to avoid repeatedly calculate the base address of timer2, and make it
easier to recognize timer1. Hope to make the next patch looks more clear.

No functional change.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200918132237.3552-6-thunder.leizhen@huawei.com