ram: k3-ddrss: Introduce support for AM642 SoCs

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>
diff --git a/drivers/ram/Kconfig b/drivers/ram/Kconfig
index 4e4e7a9..a79594d 100644
--- a/drivers/ram/Kconfig
+++ b/drivers/ram/Kconfig
@@ -62,6 +62,9 @@
 	depends on K3_DDRSS
 	prompt "K3 DDRSS Arch Support"
 
+	default K3_J721E_DDRSS if SOC_K3_J721E
+	default K3_AM64_DDRSS if SOC_K3_AM642
+
 config K3_J721E_DDRSS
 	bool "Enable J721E DDRSS support"
 	help
@@ -72,6 +75,16 @@
 	  Enabling this config adds support for the DDR memory controller
 	  on J721E family of SoCs.
 
+config K3_AM64_DDRSS
+	bool "Enable AM64 DDRSS support"
+	help
+	  The AM64 DDR subsystem comprises DDR controller, DDR PHY and
+	  wrapper logic to integrate these blocks in the device. The DDR
+	  subsystem is used to provide an interface to external SDRAM
+	  devices which can be utilized for storing program or data.
+	  Enabling this config adds support for the DDR memory controller
+	  on AM642 family of SoCs.
+
 endchoice
 
 config IMXRT_SDRAM