clk/exynos5420: assign dout_pixel id to pixel clock divider

dout_pixel is a new ID allocated for pixel clock divider. It is
queried in the driver to pass as the parent to hdmi clock while
switching between parents.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>