qemu-log: add log category for MMU info

Running barebox on qemu-system-mips* with '-d unimp' overloads
stderr by very very many mips_cpu_handle_mmu_fault() messages:

  mips_cpu_handle_mmu_fault address=b80003fd ret 0 physical 00000000180003fd prot 3
  mips_cpu_handle_mmu_fault address=a0800884 ret 0 physical 0000000000800884 prot 3
  mips_cpu_handle_mmu_fault pc a080cd80 ad b80003fd rw 0 mmu_idx 0

So it's very difficult to find LOG_UNIMP message.

The mips_cpu_handle_mmu_fault() messages appear on enabling ANY
logging! It's not very handy.

Adding separate log category for *_cpu_handle_mmu_fault()
logging fixes the problem.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Acked-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1418489298-1184-1-git-send-email-antonynpavlov@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>