drm/exynos: dsi: rename pll_clk to sclk_clk

This patch renames pll_clk to sclk_clk. The clock referenced by pll_clk
is actually not the pll input clock for dsi. The pll input clock comes
from the board's oscillator directly. But for the backward
compatibility, the old clock name "pll_clk" is also OK.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>