clocksource: Fix type confusion for clocksource_mmio_readX_Y

The types' definations are:
   o  cycle_t       -> u64
   o  readl_relaxed -> u32
   o  readw_relaxed -> u16

So let clocksource_mmio_readX_Ys return a cast to cycle_t, though
this maybe look reduntant sometimes, it make sense and they will be
more readable and less confusion...

This patch clarifies the functions type and fix it.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>