OvmfPkg: AcpiTimerLib: Use global variable during PEI_CORE and PEIM
    
    Since in OVMF both PEI_CORE and PEIM run from RAM, and thus may
    utilize global variables, use the "Base" AcpiTimerLib instance
    (instead of BaseRom) to take advantage of the improved efficiency
    of storing the timer register IO address in a global variable.
    
    This leaves only SEC using the BaseRomAcpiTimerLib instance.
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
    Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
    
    git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16377 6f19259b-4bc3-4df7-8a09-765794883524