docs(drtm): mention DRTM_SUPPORT as an experimental build option

In spite of the fact that makefile [1] indicates that DRTM_SUPPORT is
an experimental feature, it is better to mention the same in the
documentation of the build option as well.

[1]:
https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/tree/Makefile#n897

Change-Id: Ibfa328ec8ed685ce715d144d979ba37e4f49f82e
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed