arm64: dts: rockchip: fix analog headphone output on rk3399-firefly

The analog headphone jack on this board has never produced any sound
with a mainline kernel.

GPIO4_C5 has to be high for audio to reach the jack. The board file
muxes the pin to GPIO through the rt5640_hpcon pinctrl group and
references that group from the codec node, but nothing ever requests or
drives the line.

Describe it the way rk3399-roc-pc-plus.dts already does something similar,
as a simple-audio-amplifier auxiliary component, so that DAPM asserts the
GPIO while the headphone path is active.

Measured on a Firefly-RK3399 by toggling only this line while a 1 kHz
tone kept playing, with the playback PCM in state: RUNNING throughout:

  high -> audible, low -> silent, high -> audible

With this change the line is claimed and follows the audio path:

  pin 149 (gpio4-21): headphones-amp gpio4:149
  idle:    gpio-21 (|enable) out lo
  playing: gpio-21 (|enable) out hi

Fixes: 171582e00db1 ("arm64: dts: rockchip: add support for firefly-rk3399 board")
Assisted-by: LLM
Signed-off-by: Yaozhong Li <yaozhonguwl@gmail.com>
Link: https://patch.msgid.link/20260921063431.103-2-yaozhonguwl@gmail.com
Signed-off-by: Heiko Stuebner <heiko@sntech.de>

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