arm64: dts: rockchip: Add frl-enable-gpios to rk3576-luckfox-core3576

arm64: dts: rockchip: Add frl-enable-gpios to rk3576-luckfox-core3576

The board exposes the GPIO4_C6 pin to control the voltage bias on the
HDMI data lines.  It must be asserted when operating in HDMI 2.1 FRL
mode and deasserted for HDMI 1.4/2.0 TMDS mode.

Wire up the hdmi node to its dedicated GPIO via frl-enable-gpios to
allow adjusting the bias when transitioning between TMDS and FRL modes.

Additionally, remove the now unnecessary workaround of using
vcc_5v0_hdmi as hdmi-pwr-supply solely to drive the GPIO into its
default state.

Also rename the hdmi_con_en pinctrl to hdmi_tx_on_h to match the
schematic naming.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://patch.msgid.link/20260428-dts-rk-frl-enable-gpios-v2-2-924df9db884a@collabora.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>