feat(drtm): introduce drtm dynamic launch function

feat(drtm): introduce drtm dynamic launch function

This function is placeholder for checking all the necessary conditions
before doing drtm dynamic launch.
In this patch following conditions are checked (based on Table 31 of
DRTM spec beta0), rest of the conditions will be added in later
patches.
 - Only boot PE is online
 - Caller execution state is AArch64
 - Caller exception level is NS-EL2 or NS-EL1

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I622b946bc191bb39f828831336ceafbc10834c19