lib: genalloc: Use 64 bit types for holding allocations

genalloc may be used to allocate physical addresses in addition
to virtual addresses. On LPAE based systems, physical addresses
are 64-bit which does not fit in an unsigned long. Change the
type used for allocation internally to be 64 bit to ensure that
genalloc can properly allocate physical addresses > 4GB.

Ideally this will just be a temporary workaround until either
a proper fix comes elsewhere or an alternative API is used to
manage physical memory.

Change-Id: Ib10b887730e0c6916de5d1e6f77e771c6cde14bb
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>