Platform/ARM/N1Sdp: Fix RemoteDdrSize cast

RemoteDdrSize calculation wraps around when booting N1Sdp in
multichip mode. Casting it to UINT64 to fix the issue.

Signed-off-by: sahil <sahil@arm.com>
Change-Id: Ic51269a8d67669684a5f056701cfbef6beb23da2