juno:Fix CSS_USE_SCMI_SDS_DRIVER=0 configuration

A previous commit 89f2e589 ("plat/arm: remove weak implemention of
'plat_arm_psci_override_pm_ops' function") introduced a problem with the
CSS_USE_SCMI_SDS_DRIVER configuration. In juno_pm.c the
css_scmi_override_pm_ops function was used regardless of whether the
flag was set. This patch ifdefs the function to restore the original
behaviour.

Change-Id: I508025ba70cf3a9250cc6270c1df209179c37ae7
Signed-off-by: Joel Hutton <Joel.Hutton@Arm.com>