MAINTAINERS: arm/amlogic: add designated reviewers

It's already been the case for some time that Neil, Jerome and Martin
are doing the bulk of the important reviewing.  Update MAINTAINERS to
reflect reality.

Link: https://lore.kernel.org/r/20200724184107.24600-1-khilman@baylibre.com


Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>