MAINTAINERS: add Rust FW abstractions to FIRMWARE LOADER

Add missing file path of the Rust abstractions to the maintainers entry,
until we can move it to 'drivers/base/firmware_loader/'.

CC: Luis Chamberlain <mcgrof@kernel.org>
CC: Russ Weight <russ.weight@linux.dev>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Danilo Krummrich <dakr@redhat.com>
Link: https://lore.kernel.org/r/20240619132029.59296-2-dakr@redhat.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>