drivers: led: lp5569: implement write_channels api

The lp5569 has multiple pwm outputs, and thus implementing the
write_channels api to set multiple values in a single call makes sense.

Implement the write_channels function with a basic range check on the
channel range.
Since the lp5569 supports auto-increment, all of the channels can be
written in one i2c transfer, starting from the pwm register of the
start_channel.

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