cortex_a: fix lockup when writing to high address

cortex_a: fix lockup when writing to high address

On a processor with caches, when you write data to memory OpenOCD invalidates
the cache lines affected. If you write to an address within 64 bytes of
UINT32_MAX, then the for loop control variable wrapped around resulting in an
infinite loop. Change control variable to be an offset from the address
involved. We should never be asked to write 2^32 bytes, so wraparound should
not be a problem.

Change-Id: Ibfe654113eff71684862ff651e7a1cd05ccc6760
Signed-off-by: Seth LaForge <sethml@google.com>
Reviewed-on: http://openocd.zylin.com/2126
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>