Platform/AMD/OverdriveBoard: fix byte order of default MAC addresses

The PCDs containing the default MAC addresses are of type UINT64,
and so the byte order needs to be inverted. As they are currently,
both default MAC addresses are invalid since they have the multicast
bit set.

For readability, let's switch to a VOID* type PCD while at it.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>