uart: pl011: Introduce register accessor

Introduce register accessor to ease loop up table access
in later patch.
Signed-off-by: NJun Nie <jun.nie@linaro.org>
Reviewed-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>