[coreboot] UART on Apollo Lake

[Gailu Singh]

Hi Experts,

Are UARTs available on Apollo Lake different than traditional 8250 UART? Or
any specific settings (jumper etc) required to use them?

Problem Statement:
I am using UART available (through micro USB port) on Leafhill CRB. I am
able to get coreboot logs but grub fails to detect these uart.

I have following in my grub2.cfg
serial --speed=115200 --unit=0 --word=8 --parity=no --stop=1

Grub output an error
serial port COM0 not found

If I change --unit=0 to 1 or 2 grub2 reports similar error

serial port COM1 not found
serial port COM2 not found
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