Update link to SMCCC specification

The feature overview page points to the revision B published in November
2016, update the link to point to the latest version (currently 1.2
published in March 2020).

Remove the link from implementing-tests.rst, as it is not referenced in
the page.

Change-Id: I9ac12eba39d33914bfc2474a5a9851e628010349
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>