ARM: dts: rename the clock of MIPI DSI 'pll_clk' to 'sclk_mipi'

The clock which was named as 'pll_clk' is actually not the clock source
of PLL in MIPI DSI. This patch fixes this disagreement.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>