Upgrade linux-postmarketos-allwinner to 5.9 (megi's tree)

See #735 (comment 422188999) for reasoning.

Megi's tree doesn't have support for the "dontbeevil" devboard, so I've added related commits as patches. Same with support for the old pinetab panel. Other patches are the patches that we had previously, but reformatted for consistency.

Test plan:

• https://wiki.postmarketos.org/wiki/Kernel_upgrade_testing

TODO (see comments below for details):

• fix HDMI
• make sure everything in above kernel upgrade test document works
• enable CONFIG_ZRAM (#807 (closed))
• enable pinetab related kernel configs and patches (see !1627 (closed))
• fix audio
• fix camera
• test if headphone channels are swapped again, if so revert !1556 (merged)

Testing on devices:

• pinephone (@ollieparanoid: 1.2a)
• pinephone development board "dontbeevil"
• pinetab with old display (early developer version)
• pinetab with new display (sold as Early Adopter version)

• Fixes: #735 (closed), #439 (closed), #829 (closed)
• Closes: #807 (closed)