clocksource/drivers/dw_apb_timer: Fix apbt_readl return types

On Marvell BG4CT platform, we observed the __apbt_read_clocksource()
return wrong value: Let's assume the APBTMR_N_CURRENT_VALUE value is
0xf0000000, we got 0xffffffff0fffffff, but it should be 0xfffffff.

This issue should be common on all 64bit platforms. We fix the issue
by letting aptb_readl() return u32. apbt_writel() is also updated
to write u32 val rather than unsigned long.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>