ArmVirtualizationPkg: add device tree based PL011 SerialPortLib

ArmVirtualizationPkg: add device tree based PL011 SerialPortLib

This adds 2 implementations of SerialPortLib for device tree based platforms
using a PL011 UART:
- an 'early' one which is completely stateless and uses only fixed PCDs
- a normal one which takes its base address from a HOB containing the base
  address discovered in the PEI phase

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-with-remarks-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16140 6f19259b-4bc3-4df7-8a09-765794883524