mb/lenovo/{t60,r60}: Add ThinkPad R60 support as variant board

- This port should be Reclaim Your Freedom compliant
  (not certified yet).
- Untested on boards with external Radeon graphics adapter.
- Some columns on the left-most side of display are completely
  black on 1400x1050 IPS display[1]. Display works fine on Linux.
  I don't know why it appears like that. So far it has been observed
  only with native graphics initialization.
- Only GRUB2 and SeaBIOS payloads tested for now.
- 2504 docking station USB doesn't work under Linux.
  Can detect pendrive in GRUB2 payload.
- Sometimes it takes 20s of "pretending it's powered off" to run
  coreboot code. Issue is payload agnostic.
  Probably caused by missing one capacitor on my unit.

[1] https://imgur.com/a/0wpMGsm

Change-Id: Ibd9208a5eafd228f8eedbc8fb4f4eb9ed1932a14
Signed-off-by: Maciej Matuszczyk <maccraft123mc@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35864
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>

Documentation/mainboard/index.md

@@ -65,6 +65,7 @@ The boards in this section are not real mainboards, but emulators.
 
 - [Mainboard codenames](lenovo/codenames.md)
 - [Hardware Maintenance Manual of ThinkPads](lenovo/thinkpad_hmm.md)
+- [R60](lenovo/r60.md)
 - [T4xx common](lenovo/t4xx_series.md)
 - [X2xx common](lenovo/x2xx_series.md)