fix(errata): workaround for Rainier erratum 1868343

Rainier CPU is based on Neoverse N1 R4P0 version which exhibits
the erratum 1868343. This patch inherits the workaround from
neoverse_n1.S file into rainier.S file for erratum 1868343.

Signed-off-by: Manoj Kumar <manoj.kumar3@arm.com>
Change-Id: I735595229716a77d26369943086de08384cafa70