pinctrl: mvebu: a38x: Add UART1 muxing options

The MPP19 and MMP20 pins also have the ability to be muxed to the uart1
function.

Add this case to the pinctrl driver.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>