clk: vt8500: Add support for WM8750/WM8850 PLL clocks

This patch adds support for the new PLL module found in WM8750 and
WM8850 SoCs.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Mike Turquette <mturquette@linaro.org>