hw/mips: implement GIC Interval Timer

The interval timer is similar to the CP0 Count/Compare timer within
each processor. The difference is the GIC_SH_COUNTER register is global
to the system so that all processors have the same time reference.

To ease implementation, all VPs are having its own QEMU timer but sharing
global settings and registers such as GIC_SH_CONFIG.COUTNSTOP and
GIC_SH_COUNTER.

MIPS GIC Interval Timer does support upto 64 bits of Count register but
in this implementation it is limited to 32 bits only.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
