Introduce support for the AM64 DDRSS controller which uses the 16bit
variation of the controller. This controller shares much functionality
with the existing J721e support, so this patch introduces only the new
code needed for am64 specific support from "_16bit_" files with headers
under "16bit/" include path/.
Also add a CONFIG_K3_AM64_DDRSS option to the choice required for use
with CONFIG_K3_DDRSS to allow selecting AM64 support.
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
