arm64: dts: qcom: hamoa-iot-evk: Add backlight support for eDP panel

arm64: dts: qcom: hamoa-iot-evk: Add backlight support for eDP panel

The backlight on the Hamoa IoT EVK is controlled through a PWM signal.
Aligned with other x1e80100-based platforms: the PWM signal is controlled
by PMK8550, and the backlight enable signal is handled by PMC8380.

Describe the backlight device and connect it to the eDP panel to allow
for brightness control.

Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Yongxing Mou <yongxing.mou@oss.qualcomm.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Reviewed-by: Konrad Dybcio <konrad.dybcio@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20251112-hamoa_dvt_backlight-v3-1-f35b44af7fc4@oss.qualcomm.com
Signed-off-by: Bjorn Andersson <andersson@kernel.org>