arm64: dts: rockchip: Add user button to Radxa E20C

arm64: dts: rockchip: Add user button to Radxa E20C

Radxa E20C has two buttons, one SARADC maskrom button and one GPIO user
button.

Add support for the user button using a gpio-keys node.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Link: https://lore.kernel.org/r/20250304201642.831218-3-jonas@kwiboo.se
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
[ upstream commit: ad8afc8813567994164f2720189c819da8c22b99 ]

(cherry picked from commit 6793b56b79df26ab3323e5293b97577d0786ddb3)
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>