arm: extend pfn_valid to take into account freed memory map alignment

When unused memory map is freed the preserved part of the memory map is
extended to match pageblock boundaries because lots of core mm
functionality relies on homogeneity of the memory map within pageblock
boundaries.

Since pfn_valid() is used to check whether there is a valid memory map
entry for a PFN, make it return true also for PFNs that have memory map
entries even if there is no actual memory populated there.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Tested-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: Tony Lindgren <tony@atomide.com>