j721e: schema: yaml: Add general schema and J721E board config files

j721e: schema: yaml: Add general schema and J721E board config files

Schema file in YAML must be provided in board/ti/common for validating
input config files and packaging system firmware. The schema includes
entries for rm-cfg, board-cfg, pm-cfg and sec-cfg.

Board config files must be provided in board/ti/<devicename> in YAML.
These can then be consumed for generation of binaries to package system
firmware. Added YAML configs for J721E in particular.

Signed-off-by: Tarun Sahu <t-sahu@ti.com>
[n-francis@ti.com: prepared patch for upstreaming]
Signed-off-by: Neha Malcom Francis <n-francis@ti.com>