ppc: qemu: Create a virtual memory mapping of the platform bus

ppc: qemu: Create a virtual memory mapping of the platform bus

QEMU ppce500 target can dynamically instantiate an eTSEC device on
a platform bus if "-device eTSEC" is given to QEMU. It is presented
as a "simple-bus" in the device tree, with an additional compatible
string "qemu,platform".

Let's create a virtual memory mapping for it in misc_init_r(), in
preparation to adding eTSEC support.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>