msm: board-8064: Fix overflow in memory calc.

An overflow occurs in locate_unstable_memory() when physical
memory address space ends at 0xFFFFFFFF. Change code to
handle overflow by ensuring that variables are limited to
0xFFFFFFFF and by rearranging additions and subtractions.

Change-Id: I11cb890c09ef18fe7c103cbdf1232677704fcc17
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>