doc: board: Move j721e document to doc/board/ti/ directory

Move j721e document from board/ti/j721e/README to
doc/board/ti/j721e_evm.rst after converting it to RST format.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210721155849.20994-20-kishon@ti.com