SbsaQemu: Add new ACPI driver and FDT parser to count CPUs

- Add a new ACPI driver for the SbsaQemu platform which would
  handle any modifications needed for the ACPI tables.

- Add a parser function in this driver which parses the FDT created
  by Qemu to determine the number of CPUs and hence update the
  PcdCoreCount variable.

Signed-off-by: Tanmay Jagdale <tanmay.jagdale@linaro.org>
Reviewed-by: Leif Lindholm <leif@nuviainc.com>