board: ti: am65x: Update README to add R5 build support

Update the README file to add r5 build support and system
firmware support.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
1 file changed