drivers: led: lp5569: add enable-gpios

The lp5569 features a double functioning pin for enable and pwm control.
The chip, however, uses the first rising edge to initialize and get ready
for i2c communication, and then the pin alters function to pwm mode.

Add support for providing enable-gpios to the lp5569 in the dts, which
will make sure to assert the pin and wait for the chip to initialize
before attempting further configuration of the chip.

Signed-off-by: Emil Juhl <emdj@bang-olufsen.dk>