clk: samsung: exynos5433: Add CLK_IGNORE_UNUSED flag for AUD UART

This patch adds CLK_IGNORE_UNUSED flag for sclk_aud_uart gate
clock for uart3 operation.

Signed-off-by: Beomho Seo <beomho.seo@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
[s.nawrocki@samsung.com: edited the patch's summary]
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>