feat(rdn2): introduce accessor function to obtain datastore

In order to provide the EL3 SPMC a sufficient datastore to
record memory descriptor, introduce an accessor function
so that the backing memory can be allocated in a platform
defined manner to accommodate memory constraints and
desired usecases.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: If5842e79c2ad22ccc17362b114f47d9900d82f7e
1 file changed