FatPkg: Use BasePcdLibNull to prevent PCD dependency

FatPkg: Use BasePcdLibNull to prevent PCD dependency

When using DxePcdLib or PeiPcdLib, a PCD protocol or
PPI dependency is added.

This dependency should not be required since the FAT
drivers use fixed PCD values.

Signed-off-by: jljusten
Reviewed-by: mdkinney
(based on FatPkg commit b098f260669c4f7856b8cf01fff2bbeb206f22c2)

[jordan.l.justen@intel.com: Use script to relicense to 2-clause BSD]
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Mark Doran <mark.doran@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>