ArmVirtualizationPkg: introduce QemuFwCfgLib instance for DXE drivers

After reviewing OvmfPkg's use of its own QemuFwCfgLib instances, it is
clear that its only pre-DXE fw_cfg dependency concerns S3 support (the
QemuFwCfgS3Enabled() call in "PlatformPei/Platform.c").

For ARM guests, S3 is in the distant future, but we can see several
shorter term applications for fw_cfg that all reside in DXE:
- controlling boot order (to be implemented in PlatformBdsLib for Intel
  BDS),
- supporting -kernel / -initrd / -append boot on QEMU (to be implemented
  in PlatformBdsLib for Intel BDS, similarly),
- loading and linking ACPI tables,
- installing SMBIOS tables.

Therefore it makes sense to add a simple MMIO-based fw_cfg client library
to ArmVirtualizationPkg that for the moment is only available to
DXE_DRIVER modules.

Because MMIO accesses are costly on KVM/ARM, InternalQemuFwCfgReadBytes()
accesses the fw_cfg data register in full words. This speeds up transfers
almost linearly.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16567 6f19259b-4bc3-4df7-8a09-765794883524

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