cpus: denver: Implement static workaround for CVE-2018-3639

For Denver CPUs, this approach enables the mitigation during EL3
initialization, following every PE reset. No mechanism is provided to
disable the mitigation at runtime.

This approach permanently mitigates the EL3 software stack only. Other
software components are responsible to enable it for their exception
levels.

TF-A implements this approach for the Denver CPUs with DENVER_MIDR_PN3
and earlier:

*   By setting bit 11 (Disable speculative store buffering) of
    `ACTLR_EL3`

*   By setting bit 9 (Disable speculative memory disambiguation) of
    `ACTLR_EL3`

TF-A implements this approach for the Denver CPUs with DENVER_MIDR_PN4
and later:

*   By setting bit 18 (Disable speculative store buffering) of
    `ACTLR_EL3`

*   By setting bit 17 (Disable speculative memory disambiguation) of
    `ACTLR_EL3`

Change-Id: If1de96605ce3f7b0aff5fab2c828e5aecb687555
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>