arm64: dts: rockchip: Add uart0 pinctrl to Radxa E20C

arm64: dts: rockchip: Add uart0 pinctrl to Radxa E20C

Radxa E20C route UART0 M0 pins (GPIO4_C7 and GPIO4_D0) to the onboard
CH340B for debug console use.

Add pinctrl for UART0 M0 pins used for serial console.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Link: https://lore.kernel.org/r/20250228064024.3200000-6-jonas@kwiboo.se
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[ upstream commit: 0d2312f0d3e4ce74af0977c1519a07dfc71a82ac ]

(cherry picked from commit 9bcf6ccdd87c3be48fe7d75150c6e403c5c0a42d)
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>