uart: pl011: Introduce register look up table

Introduce register look up table as different SOC venders
may have different register offset for the some register.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>