arm64: Allow CAVIUM_TX2_ERRATUM_219 to be selected

arm64: Allow CAVIUM_TX2_ERRATUM_219 to be selected

BugLink: https://bugs.launchpad.net/bugs/1850456
commit 603afdc9438ac546181e843f807253d75d3dbc45 upstream.

Allow the user to select the workaround for TX2-219, and update
the silicon-errata.rst file to reflect this.

Cc: <stable@vger.kernel.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>