pinctrl: sunxi: Add UART pinmuxes

This includes UART0 and R_UART (s_uart) on all supported platforms, plus
the additional UART configurations from arch/arm/mach-sunxi/board.c.

Pin lists and mux values were taken from the Linux drivers.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>