OvmfPkg: extract some bits and port offsets common to Q35 and I440FX

OvmfPkg: extract some bits and port offsets common to Q35 and I440FX

The PMBA_RTE and ACPI_TIMER_OFFSET macros apply equally to both boards,
plus they are triplicated between the various AcpiTimerLib instances.
Define them centrally in "OvmfPlatforms.h".

Cc: Gabriel Somlo <somlo@cmu.edu>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Acked-by: Gabriel Somlo <somlo@cmu.edu>
Tested-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@17436 6f19259b-4bc3-4df7-8a09-765794883524