m32r: define CPU_BIG_ENDIAN

The build of m32r allmodconfig is giving lots of build warnings about:

  include/linux/byteorder/big_endian.h:7:2:
	warning: #warning inconsistent configuration,
		needs CONFIG_CPU_BIG_ENDIAN [-Wcpp]
	#warning inconsistent configuration, needs CONFIG_CPU_BIG_ENDIAN

Define CPU_BIG_ENDIAN like the way CPU_LITTLE_ENDIAN is defined.

Link: http://lkml.kernel.org/r/1505678083-10320-1-git-send-email-sudipm.mukherjee@gmail.com
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>