SbsaQemu: SbsaQemu.dsc: Move CoreCount and Fdtlib

- Since the core count is dynamic and controlled by Qemu, move the
  PcdCoreCount from [PcdsFixedAtBuild] to [PcdsDynamic] section.

- Move FdtLib from [LibraryClasses.common.PEIM] to [LibraryClasses.common]
  section so that driver DXEs can use the device tree APIs.

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