malta: update MIPS64 ISA to R2

malta: update MIPS64 ISA to R2

Usage of current R1 ISA is inconsistent with the MIPS32 subtarget, little
used and has limited utility for testing.

Many distros target a minimum R2 ISA. Debian MIPS 32-bit/64-bit ports all
use MIPS R2 ISA since Stretch, for example. Fedora's MIPS arch also targets
the R2 ISA for 32-bit/64-bit.

Widely used MIPS64 platforms like Octeon are based on the MIPS R2 ISA or
later, and benefit from having a compatible test platform in OpenWRT.

While Linux does support MIPS64 R1 targets, its usefulness for development
and testing is limited. As an example, the modern Linux eBPF JIT requires
a MIPS R2 ISA or later.

Signed-off-by: Tony Ambardar <itugrok@yahoo.com>
[Refresh config and fix README]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>