ARM i.MX6: remove gate_mask from pllv3

Now that the additional enable bits in the enet PLL are handled
as gates, the gate_mask is identical for all plls. Remove the
gate_mask from the code and use the BM_PLL_ENABLE bit for
enabling/disabling the PLL.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>