hw/a15mpcore.c: Add Cortex-A15 private peripheral model

hw/a15mpcore.c: Add Cortex-A15 private peripheral model

Add a model of the Cortex-A15 memory mapped private peripheral
space. This is fairly simple because the only memory mapped
bit of the A15 is the GIC.

Note that we don't currently model a VGIC and therefore don't
map the VGIC related bits of the GIC.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>