riscv: dts: thead: Add TH1520 pinctrl settings for UART0

Add pinctrl settings for UART0 used as the default debug console on
both the Lichee Pi 4A and BeagleV Ahead boards.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Thomas Bonnefille <thomas.bonnefille@bootlin.com>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>