msm: board-8960: 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: Ie2b5f2ce0a6ddc6cdffa7554957bd458cf744e5d
Signed-off-by: Jack Cheung <jackc@codeaurora.org>