[POWERPC] 4xx: Change dts files to support jumbo frames

This patch changes the "max-frame-size" property to 9000 for all gbit
enabled 4xx boards. All those ports generally support jumbo frames, so
let's give the user a chance to enable it.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>