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/bcm968580xref.dts b/arch/arm/dts/bcm968580xref.dts
index a034e38..6d787bd 100644
--- a/arch/arm/dts/bcm968580xref.dts
+++ b/arch/arm/dts/bcm968580xref.dts
@@ -8,8 +8,8 @@
 #include "bcm6858.dtsi"
 
 / {
-	model = "Broadcom bcm68580xref";
-	compatible = "broadcom,bcm68580xref", "brcm,bcm6858";
+	model = "Broadcom BCM968580xref Reference Board";
+	compatible = "brcm,bcm968580xref", "brcm,bcm6858", "brcm,bcmbca";
 
 	aliases {
 		serial0 = &uart0;