ArmVirtualizationPkg: move early UART discovery to PlatformPeim

This is partially motivated by the desire to use PrePi in a virt
environment, and in that configuration, ArmPlatformInitializeSystemMemory()
is never called. But actually, this is a more suitable place anyway.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16958 6f19259b-4bc3-4df7-8a09-765794883524