dts: vt8500: Correct reference clock on WM8850 SoCs

WM8850 SoCs use a 24Mhz reference clock for the PLLs but the SoC file
currently parents all PLLs to the 25Mhz reference clock.

This patch corrects the PLL parent clock references.

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