feat(neoverse-rd): allow RESET_TO_BL31 for third gen platforms

Allow building RESET_TO_BL31 for third generation neoverse-rd
platforms.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Signed-off-by: Rakshit Goyal <rakshit.goyal@arm.com>
Change-Id: I30256969e5671043b3e58c76922985f7252429af