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 <[e-mail]>
Signed-off-by: Thomas Bogendoerfer <[e-mail]>

MAINTAINERS

···
 F:	Documentation/mips/
 F:	arch/mips/
 F:	drivers/platform/mips/
+F:	include/dt-bindings/mips/
 
 MIPS BOSTON DEVELOPMENT BOARD
 M:	Paul Burton <[e-mail]>