clocksource/drivers/dw_apb_timer: Inline apbt_readl and apbt_writel

It seems gcc can automatically inline apbt_writel() for us, but
apbt_real isn't inlined. This patch makes them inline to get a trivial
performance improvement: 4096 rounds of __apbt_read_clocksource() call
spend time on Marvell BG4CT platform:

 before the patch 1275240ns on average
 after the patch 1263240ns on average

so we get 1% performance improvement.

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