lib: No need to provide default PMP region using platform callbacks

The default (usually last) PMP region provides S-mode access to
complete memory range not covered by other PMP regions.

Currently, the default PMP region is described as platform specific
PMP region which is not appropriate because all platforms need it
and default PMP region should be part of generic library.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>