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>