MAINTAINERS: Add LP8788 MFD driver entry

Cc: Anton Vorontsov <cbou@mail.ru>
Cc: Bryan Wu <cooloney@gmail.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>