docs(rme): add instruction to build rmm

Add documentation to build and run TF-A with RMM,
Linux kernel and TFTF Realm Payload.

Signed-off-by: Shruti Gupta <shruti.gupta@arm.com>
Change-Id: I951b41a144aabe0fec16eb933d7f005a65f06fb2
1 file changed