ArmVirtPkg/FdtClientDxe: report address and size cell count directly

ArmVirtPkg/FdtClientDxe: report address and size cell count directly

The FDT client protocol methods dealing with "reg" properties return
the size of a "reg" element. Currently, we have hardcoded this as '8',
since #address-cells == #size-cells == 2 in most cases. However, for
different values, have a single 'reg' element size is not unambiguous,
since - however unlikely - if #address-cells != #size-cells, we do not
know which is which.

So before adding more methods to the protocol, fix up this oversight.

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