vlib: add max-size configuration parameter for pmalloc

Enable 'max-size' configuration parameter to set maximum size of
memory space allocated for pmalloc module. This parameter along
with existing 'base-addr' falls into physmem section. Both are
dedicated parameters to pmalloc module.

Type: feature

Change-Id: I939f25156b9a43440a24d69a7397d57eb6c2d392
Signed-off-by: Jieqiang Wang <Jieqiang.Wang@arm.com>