microblaze: Prevent the overflow of the start

[ Upstream commit 061d2c1d ]

In case the start + cache size is more than the max int the
start overflows.
Prevent the same.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>