dts: clk: vt8500: Update SoC dtsi to use WM8850 PLL

 clocks

Change the WM8850 SoC dtsi to use the new wm8850 specific PLL clock
binding. Previously, the WM8850 used the wm8750 pll clock which is
actually different to the wm8850 pll clock.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>