ArmVirtPkg/PlatformBootManagerLib: introduce the progress bar

ArmVirtPkg/PlatformBootManagerLib: introduce the progress bar

ArmVirtPkg's Platform BDS has never had a progress bar. We can easily add
one, by copying the PlatformBootManagerWaitCallback() function verbatim
from

  Nt32Pkg/Library/PlatformBootManagerLib/PlatformBootManager.c

It can be tested by passing the following option to QEMU (5 seconds):

  -boot menu=on,splash-time=5000

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>