powerpc: Enable additional BAT registers in setup_745x_specifics()

Currently the kernel expects the additional four IBAT and DBAT registers
to be available, but doesn't enable these registers on 745x CPUs, which
have them disabled after reset. Thus set the HIGH_BAT_EN bit in HID0
register, if the corresponding MMU feature is defined.

Signed-off-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>