MIPS: asm: spram: Add new symbol for MIPS scratch pad storage

MIPS R6, just like MIPS R2, have scratch pad storage, so add a new
symbol which is selected by MIPS R2 and R6.

Link: http://www.linux-mips.org/archives/linux-mips/2015-01/msg00389.html
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>

diff --cc arch/mips/Kconfig
Simple merge

diff --cc arch/mips/include/asm/spram.h
Simple merge

diff --cc arch/mips/kernel/Makefile
Simple merge