ram: Introduce K3 AM654 DDR Sub System driver

K3 based AM654 devices has DDR memory subsystem that comprises
Synopys DDR controller, Synopsis DDR phy and wrapper logic to
intergrate these blocks into the device. This DDR subsystem
provides an interface to external SDRAM devices. Adding support
for the initialization of the external SDRAM devices by
configuring the DDRSS registers and using the buitin PHY
routines.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Schuyler Patton <spatton@ti.com>
Signed-off-by: James Doublesin <doublesin@ti.com>