drivers: led: lp50xx: add enable gpio

Some boards may have connected the enable pin of the chipset to a GPIO.
On these boards, it is necessary to configure and set this GPIO before
using the chipset, otherwise the I2C circuitry is disabled.

Based on initial work from:
  - Marek Janus <marek.janus@grinn-global.com>
  - Rico Ganahl <rico.ganahl@bytesatwork.ch>

Signed-off-by: Mathieu Anquetin <mathieu.anquetin@groupe-cahors.com>