drivers: led: lp50xx: add reset

The LP50XX family has a specific register to reset the configuration to
default state from any other state. Use this instead of relying on the
manual configuration of registers during startup.

Based on initial work from: Marek Janus <marek.janus@grinn-global.com>

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