cpu: mips: Add support for mips32r2

 cpu/mips32r2_common adds base architecture support for mips32r2 cores it can be
 built in it own right as a 'CPU', but is dependant on a bootloader (like
 u-boot) to have bootstrapped the system, this has been tested on a 'malta'
 FPGA system (BOARD=mips-malta) with various mips32r2 compliant cores
 (interAptiv, P5600, etc).