MAINTAINERS: add include/dt-bindings/mips to MIPS

Maintainers of the directory Documentation/devicetree/bindings/mips
are also the maintainers of the corresponding directory
include/dt-bindings/mips.

Add the file entry for include/dt-bindings/mips to the appropriate
section in MAINTAINERS.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>