arm: handle discontiguous page structures between sections

If SPARSEMEM is enabled and there is a large amount of
memory, the page structures for the various sections
may not be contiguous. The code to traverse all of the
page structures in page_init() was incorrectly assuming
all of the page structures were contiguous, causing
early kernel panics in this case.

Change-Id: I10548520f4d1c0c232a2df940ab0f9d57078c586
Signed-off-by: Larry Bassel <lbassel@codeaurora.org>