Platform/ARM: Update Morello and N1Sdp to use PrePeiCoreUniCore

Modern platforms that use TF-A should use PrePeiCoreUniCore, since
any secondary cores will be put in the TF-A holding pen and won't
reach EDK2.

Update the Morello and N1Sdp files to use PrePeiCoreUniCore instead of
PrePeiCoreMPCore.

Signed-off-by: Rebecca Cran <rebecca@quicinc.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>