clk: exynos5250: Add CLK_IGNORE_UNUSED flag for pmu clock

Currently 'pmu' clock is not handled by any of the drivers.
Also before the introduction of CCF, this clock was not defined,
hence was left enabled always.

When this clock is disabled, software reset register becomes
inaccessible and system reboot doesn't work.

Upon restoring the default behaviour, system reboot starts working.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>