Draft: qcs6490-mnt-quasar.dts porting/hacking WIP

tested on Linux quasar 6.16.12-mnt-reform-arm64 #1 SMP PREEMPT Debian 6.16.12-2+reform20250910T145641Z

what works so far:

• one i2c (i2c4)
• usb (both ports, incl. superspeed on one of them)
• pci link (but not function). both pcie0 & pcie1. pericom pcie switch is found on pcie0.
• ufs

what doesn't work:

• DSI to sn65dsi86 eDP bridge (no clue yet, but interesting that the AUX channel works, that means the main DSI clock is there. i can also check with a normal DSI panel instead of that bridge)
• DP. the kernel i have is 6.16 and doesn't have neil's MUX functions in phy-qcom-qmp-combo.c. probably should try with linux-next, i just don't have a good workflow for installing tons of modules on the target, and i think monolithic kernel is not that great with qcom? (or maybe ok?) (hints appreciated). but i also need maybe a new dts property to force a mux mode + orientation without needing glink/any actual usb-c port
• i2c6 (xbl, i can work around this until we get a new one by just wiring it to i2c4)
• spi6 (xbl too i guess?)
• pwm for backlight (i thought i could maybe do pwms = <&pm8350c_pwm 8 50000 0>; but that doesn't seem to work
• sd card (sdc2)
• ethernet (not showing up on switch, but yeah that's for me to debug, probably hw issue). using USB ethernet for now.
• can't talk to any pcie devices on any controller, even though they link up

not yet sure how to wire up in dts:

• audio (i2s to codec, like wm8960 on this board)

so yeah. either DSI or DP would be great to fix, to be able to see something ^^