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>

arch/powerpc/cpu/ppc4xx/start.S

@@ -340,6 +340,9 @@ _start_440:
 	mfspr	r1,SPRN_DBCR0
 	andis.	r1, r1, 0x8000	/* test DBCR0[EDM] bit			*/
 	bne	skip_debug_init	/* if set, don't clear debug register	*/
+	mfspr	r1,SPRN_CCR0
+	ori	r1,r1,CCR0_DTB@l /* Disable Trace Broadcast */
+	mtspr	SPRN_CCR0,r1
 	mtspr	SPRN_DBCR0,r0
 	mtspr	SPRN_DBCR1,r0
 	mtspr	SPRN_DBCR2,r0