arm: mach-k3: j721e: Add detection for j721e

Add an api soc_is_j721e(), and use it to enable certain functionality
that is available only on j721e. This detection is needed when DT is not
available.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Suman Anna <s-anna@ti.com>