ARM: dts: dra7: Add ti,secure-ram node to ocmcram1 node

Secure variants of DRA7xx and AM57xx SoCs may need to reserve a region
of the SRAM for use by secure software. To account for this, add a child
node to the ocmcram1 node that will act as a placeholder at the start
of the SRAM for the reserved region of memory that may be required
by secure services. The node is added with size 0 so that by default
parts will have the full space available but the bootloader or board dts
file is able to resize the node as needed depending on how much reserved
space is needed, if any, so end users of the ocmcram1 region on HS parts
must be aware that a smaller amount of SRAM than expected may be available.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Reviewed-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>