clk: samsung: Add CLK_IGNORE_UNUSED flag for the sysreg clocks

clk: samsung: Add CLK_IGNORE_UNUSED flag for the sysreg clocks

Currently no driver *) handles the sysreg clock, with an assumption
that this clock is always left in its default state (enabled).

Before commit fde8e7348ea39afed76645945d1efa
ARM: EXYNOS: Migrate clock support to common clock framework

the sysreg clock was not even defined and hence wasn't handled
explicitly in the kernel.

To restore the previous behaviour disable masking the sysreg clock
off in the clock core by default.

*) Except the Exynos4x12 FIMC-IS driver, which will be modified
   to not touch the sysreg clock.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>