MAINTAINERS: add Russ Weight as a firmware loader maintainer

Russ has done extensive rework on the usermode helper interface for
the firmware loader. He's also exressed recent interest with maintenance
and has kindly agreed to help review generic patches for the firmware
loader. So add him as a new maintainer!

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Russ Weight <russell.h.weight@intel.com>
Link: https://lore.kernel.org/r/20220512185529.3138310-1-mcgrof@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>