Fix: firefly-itx-3588j: Use mainline u-boot and rename audio output

[ColorfulRhino]

Description

Helping @SeeleVolleri from: #6849

The default 2017.09 u-boot for rk3588 doesn't seem to work on this board with the generic config. Try mainline u-boot.

Reference: #6834 (comment)

@SeeleVolleri Please test if this works :) You can edit this message to add your tests as well.

How Has This Been Tested?

• Test A

Checklist:

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings

[ColorfulRhino]

Closing since it was just to help in #6849