[POWERPC] Turn on tigon3 support in maple_defconfig

I think that most people who use maple_defconfig are doing so for a JS21,
so it might make sense to turn Tigon3 support on by default.

Built and booted on a JS21.

Signed-off-by: Amos Waterland <apw@us.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>