clk: meson: clk-pll: add enable bit

Add the enable the bit of the pll clocks.
These pll clocks may be disabled but we can't model this as an external
gate since the pll needs to lock when enabled.

Adding this bit allows to drop the poke of the first register of PLL.
This will be useful to model the different components of the pll using
generic clocks elements

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>