clk: imx: refine the powerdown bit of clk-pllv3

The powerdown bit is a bit confused, let's change it to power_bit
to relfect both powerdown and powerup case according to different
plls.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>