ARM: Fix bank_pfn_end macro overflow

bank_pfn_end macro overflows when physical memory space
configuration ends at 0xFFFFFFFF.

The macro adds start and size together before converting
to a page frame number. Change the macro to convert start
and size first and then add them together.

Change-Id: If091fd860e6cc94f2221164bd79bf34415819e66
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>