ppc4xx: Disable trace broadcast for 44x non debug mode

By default the trace broadcast is enabled on 44x systems.

To reduce power consumption when instruction tracing is
not needed, disable trace broadcast.

Check External Debug Mode (EDM) bit to detect if it should be
disabled or not.

Resetting system via a debugger will set the DBCR0[EDM] bit.
Resetting via u-boot or OS will not.

Signed-off-by: Victor Gallardo <vgallardo@apm.com>
Signed-off-by: Stefan Roese <sr@denx.de>