target/ppc: add external PID support

target/ppc: add external PID support

External PID is a mechanism present on BookE 2.06 that enables application to
store/load data from different address spaces. There are special version of some
instructions, which operate on alternate address space, which is specified in
the EPLC/EPSC regiser.

This implementation uses two additional MMU modes (mmu_idx) to provide the
address space for the load and store instructions. The QEMU TLB fill code was
modified to recognize these MMU modes and use the values in EPLC/EPSC to find
the proper entry in he PPC TLB. These two QEMU TLBs are also flushed on each
write to EPLC/EPSC.

Following instructions are implemented: dcbfep dcbstep dcbtep dcbtstep dcbzep
dcbzlep icbiep lbepx ldepx lfdepx lhepx lwepx stbepx stdepx stfdepx sthepx
stwepx.

Following vector instructions are not: evlddepx evstddepx lvepx lvepxl stvepx
stvepxl.

Signed-off-by: Roman Kapl <rka@sysgo.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>