clk: imx: pllv3: add enable() support

Before set_rate() pllv3 needs enable() to power the pll up.
Add enable() taking into account different power_bit and
different powerup_set, because some pll needs its power_bit to be
set or reset to be powered on.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>