powerpc: Update g5_defconfig

This updates the g5 defconfig to include nouveau instead of nvidiafb
(which works much better nowadays, in fact the latter crashes on modern
distros), and to set CONFIG_VT_HW_CONSOLE_BINDING without which takeover
from the firmware offb by nouveau doesn't work properly (and leads to
unexplained black screens for some users).

The rest is churn of going through defconfig / savedefconfig

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>