clk: samsung: exynos5250: Add CLK_IGNORE_UNUSED flag for the sysreg clock

clk: samsung: exynos5250: Add CLK_IGNORE_UNUSED flag for the sysreg clock

The sysreg (system register) generates control signals for various blocks
like disp1blk, i2c, mipi, usb etc. However, it gets disabled as an unused
clock at boot-up. This can lead to failures in operation of above blocks,
because they can not be configured properly if this clock is disabled.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
[t.figa: Updated patch description.]
Signed-off-by: Tomasz Figa <t.figa@samsung.com>