MAINTAINERS: imx: include drivers/firmware/imx path

Due to newly added IMX SCU firmware support, let's add
drivers/firmware/imx into maintainership.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>