fix(context-mgmt): enable SCXTNUM access

Enable SCXTNUM_ELx access for lower ELs in non-secure state.
Make realm context setup take this build flag into account but enable it
by default when RME is used.

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: Ieb0186b2fdffad464bb9316fc3973772c9c28cd0