leds-pca9633: Add mutex to the ledout register

To update an LED a register has to be read, updated and writen. If
another LED whas been updated at the same time, this could lead into
wrong updates.

This patch adds a common mutex to all the leds of the same chip to
protect the ledout register.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>