ID_MMFR0 has an invalid value on aarch64 cpu (A57, A53)

The ID_MMFR0 register, accessed from aarch64 state as an invalid value:
- ARM ARM v8 documentation (D7.2 General system control registers) described bits AuxReg[23:20] to be
  "In ARMv8-A the only permitted value is 0010"
- Cortex A53 and Cortex A57 TRM describe the value to be 0x10201105, so AuxReg[23:20] is 0010 too
- in QEMU target/arm/cpu64.c, the relevant value is
  cpu->id_mmfr0 = 0x10101105;

The 1 should be changed to 2.

Spotted & Tested on the following qemu revision:

commit 48ae1f60d8c9a770e6da64407984d84e25253c69
Merge: 78b62d3 b867eaa
Author: Peter Maydell <email address hidden>
Date:   Mon Oct 16 14:28:13 2017 +0100