arm: bcmbca: remove bcm6858 support under CONFIG_ARCH_BCM6858

Now that BCM6858 is supported under CONFIG_ARCH_BCMBCA and
CONFIG_BCM6858, remove the original ARCH_BCM6858 support and migrate its
configuration and dts settings. This includes:
- Remove the bcm968580xref board folder. It is replaced by the generic
bcmbca board folder.
- Update bcm968580xref board dts with the new compatible string.
- Delete broadcom_bcm968580xref.h and merge its setting to the new
bcm96858.h file.
- Remove bcm968580xref_ram_defconfig as a basic config version of
bcm96858_defconfig is now added.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>
diff --git a/arch/arm/dts/Makefile b/arch/arm/dts/Makefile
index 37659cb..546a460 100644
--- a/arch/arm/dts/Makefile
+++ b/arch/arm/dts/Makefile
@@ -1167,9 +1167,6 @@
 dtb-$(CONFIG_ARCH_BCM6753) += \
 	bcm96753ref.dtb
 
-dtb-$(CONFIG_ARCH_BCM6858) += \
-	bcm968580xref.dtb
-
 dtb-$(CONFIG_TARGET_BCMNS3) += ns3-board.dtb
 
 dtb-$(CONFIG_ARCH_BCMSTB) += bcm7xxx.dtb
@@ -1200,7 +1197,8 @@
 	bcm96856.dtb \
 	bcm968360bg.dtb
 dtb-$(CONFIG_BCM6858) += \
-	bcm96858.dtb
+	bcm96858.dtb \
+	bcm968580xref.dtb
 dtb-$(CONFIG_BCM6878) += \
 	bcm96878.dtb