arch: arm64: init VMPIDR_EL2 in z_arm64_el2_init

VMPIDR_EL2 is assigned the value returned by EL2 reads of MPIDR_EL1

MPIDR_EL1 is the register holding the Multiprocessor ID which is to
identify different cores. Because of the virtualization requirements
for AArch64, MPIDR_EL1 should be virtualized (the different virtualized
cores can run on the same physical core). Thus the value of MPIDR_EL1
should be switched when the VM is switched. Setting the VMPIDR_EL2 is
the way to change the value returned by EL1 reads of MPIDR_EL1. Even
without virtualization, we still need to set VMPIDR_EL2 during booting
at EL2 or EL3. Otherwise, all cores' IDs are zero at the EL1 stage
which will break the SMP system.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>