docs(build): add build option for DRTM support

Documented the build option for DRTM support.

Signed-off-by: Manish V Badarkhe <manish.badarkhe@arm.com>
Signed-off-by: Lucian Paul-Trifu <lucian.paultrifu@gmail.com>
Change-Id: Ic1543ee5f1d0046d5062d9744bd1a136d940b687
1 file changed