drm/exynos/decon5433: add PCLK clock

PCLK clock is used by DECON IP. The patch also replaces magic number with
number of clocks in array definition.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>