msm: board-8064: Handle unstable section overflow

If the calculation for 'high' overflows, it is set to
0xFFFFFFFF. This will cause the unstable region to be
aligned in the wrong way.

This change avoids the overflow by subtracting the
MIN_MEMORY_BLOCK_SIZE before adding the membank size.

Change-Id: Ided0557e2b1d14bbc3742d5a833674cde918d539
Signed-off-by: Jack Cheung <jackc@codeaurora.org>