powerpc/40x: remove unused "config 405GPR"

The last user of Kconfig symbol 405GPR got removed in release v3.2.
Remove this symbol too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Josh Boyer <jwboyer@gmail.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>