hw/mips: add initial Global Config Register support

Add initial GCR support to indicate number of VPs present in the system,
L2 bypass mode and revision number.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
[leon.alrae@imgtec.com:
 * removed GIC part,
 * changed commit message,
 * replaced %lx format spec. with PRIx64,
 * renamed mips_gcr.{c,h} to mips_cmgcr.{c,h},
 * replaced CONFIG_MIPS_GIC with CONFIG_MIPS_CPS]
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
