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/include/configs/bcm96858.h b/include/configs/bcm96858.h
index 4e584b4..8bd1169 100644
--- a/include/configs/bcm96858.h
+++ b/include/configs/bcm96858.h
@@ -8,4 +8,8 @@
 
 #define CONFIG_SYS_SDRAM_BASE		0x00000000
 
+#ifdef CONFIG_MTD_RAW_NAND
+#define CONFIG_SYS_MAX_NAND_DEVICE	1
+#endif /* CONFIG_MTD_RAW_NAND */
+
 #endif