configs/xilinx-ppc405-generic: Typos and size

-Fix typos (runnining -> running)
-Increase default size

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>