ARM: dts: Fix CLK_UART_ISP_SCLK clock assignment in exynos4x12.dtsi

Assign proper FIMC-IS UART gate clock in the device DT node and not
use the SRC_MASK gate. This fixes regression introduced in commit
a37c82a3

 ("clk: samsung: exynos4: Remove
SRC_MASK_ISP gates").

Without this change exynos4 fimc-is driver fails to probe with an
error log:

[    1.842447] ERROR: could not get clock /camera/fimc-is@12000000:uart(13)
[    1.848529] exynos4-fimc-is 12000000.fimc-is: failed to get clock: uart
[    1.855275] exynos4-fimc-is: probe of 12000000.fimc-is failed with error -2

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>