clk: st: Add polarity bit indication

This patch introduces polarity indication for pll power up bit
and for standby bit in order to have same code between stih416
and stih407 boards.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>