Platform: RSS: Use common CMSDK UART driver

Removes the RSS CMSDK UART driver and uses the common Arm one instead.
Also propagates a small fix from the RSS CMSDK UART driver to the Arm
one.

Signed-off-by: Jamie Fox <jamie.fox@arm.com>
Change-Id: Ifc4fc28aeeed35d47ee4acc1201fdaa742eef6f1