bcm968580: rename to bcm968580xref

The name of the board is bcm968580xref,
so rename the config to bcm968580xref too.
Signed-off-by: NPhilippe Reynes <philippe.reynes@softathome.com>

board/broadcom/bcm968580xref/MAINTAINERS

-BROADCOM BCM968580XREF
+BCM968580XREF BOARD
 M:	Philippe Reynes <philippe.reynes@softathome.com>
 S:	Maintained
 F:	board/broadcom/bcm968580xref/
 F:	include/configs/broadcom_bcm968580xref.h
-F:	configs/bcm968580_ram_defconfig
+F:	configs/bcm968580xref_ram_defconfig