UefiCpuPkg/Mplib.c: Perform complete initialization when enable AP.

UefiCpuPkg/Mplib.c: Perform complete initialization when enable AP.

PI has description said If an AP is enabled, then the implementation must
guarantee that a complete initialization sequence is performed on the AP,
so the AP is in a state that is compatible with an MP operating system.
Current implementation just set the AP to idle state when enable this AP
which is not follow spec. This patch fix it.

Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Eric Dong <eric.dong@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>