serial: 8250_dw: Add optional reset control support

The Allwinner A31 and A23 SoCs have a reset controller
maintaining the UART in reset by default.

This patch adds optional reset support to the driver.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>