MIPS: make possible board-specific header files

This patch makes possible to put a board-specific
header file (e. g. foobar.h) to arch/mips/boards/*/include/board/.

Header file usage:
 #include <board/foobar.h>

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

arch/mips/Makefile

@@ -96,6 +96,8 @@ else
 MACH  :=
 endif
 
+CPPFLAGS += -I$(BOARD)/include
+
 common-y += $(BOARD) $(MACH)
 common-y += arch/mips/lib/
 common-y += arch/mips/boot/