target-mips: Enable access to required RDHWR hardware registers

target-mips: Enable access to required RDHWR hardware registers

While running in the usermode emulator all of the required*
MIPS32r2 RDHWR hardware registers should be accessible (the
Linux kernel enables access to these same registers).  Note
that these registers are still enabled when the MIPS ISA is
not release 2.  This is OK since the Linux kernel emulates
access to them when they are not available in hardware.

* There is also the ULR register which is only recommended
  for full release 2 compliance.  Incidentally, accessing
  this register in the current implementation works fine
  without flipping its access bit.

Signed-off-by: Meador Inge <meadori@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>