arch: arm64: Fix the assertion failed when MP_NUM_CPUS >= 3

"arm64_cpu_boot_params.mpid" should be assigned to "master_core_mpid"
after secondary CPU core up.

Because "arm64_cpu_boot_params.mpid" is used to check the next up CPU
core's mpid is the excepted mpid. After excepted CPU core up, the
"arm64_cpu_boot_params.mpid" doesn't restore to primary CPU core's mpid
and then the primary CPU core try to up third CPU core will crash in
assertion.

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