Explicitly set the QEMU cpu type and support the MIPS R2 ISA, for both
64-bit and 32-bit targets. The later previously supported MIPS R2 by
implicit default.
This is needed after commit 93608697f3 ("malta: update MIPS64 ISA to R2"),
otherwise booting malta images with scripts/qemustart will hang.
Signed-off-by: Tony Ambardar <itugrok@yahoo.com>