For some clocksource devices, for example, the registers are 32-bit, while
the lower 16-bit is used for timer counting(And reading the upper 16-bit
will return 0).
For example, when the counter value is 0x00001111, and then the
~readl_relaxed(to_mmio_clksrc(c)->reg) will return the value of 0xFFFFEEEE,
but it should be 0x0000EEEE.
So just using the c->mask to mask the unused bits.
Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>