serial: amba-pl011: adopt pinctrl support

Cc: linux-serial@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>