Overview:

A Fitlet2 computer has an onboard serial port which is a PCI attached HSUART.  The uart driver does not recognize the port, no /dev/cua* is created.  

To Reproduce:

Attempt to use a serial port on a fitlet2 computer, or perhaps more generally, a Celeron N3350/Pentium N4200/Atom E3900.  Note that there is no /dev/cua* for the serial port.

The hardware appears in pciconf as:

none2@pci0:0:24:2:      class=0x118000 card=0x72708086 chip=0x5ac08086 rev=0x0b hdr=0x00
    vendor     = 'Intel Corporation'
    device     = 'Celeron N3350/Pentium N4200/Atom E3900 Series HSUART Controller'
    class      = dasp

I cargo-culted the fix in PR 207910, and added the PCI id to the list in src/sys/dev/uart/uart_bus_pci.c.

Adding the following at line 168:

{ 0x8086, 0x5abc, 0xffff, 0, "Celeron N3350/Pentium N4200/Atom E3900 Series HSUART#0", 0x10,
        24 * DEFAULT_RCLK, 2 },
{ 0x8086, 0x5abe, 0xffff, 0, "Celeron N3350/Pentium N4200/Atom E3900 Series HSUART#1", 0x10,
        24 * DEFAULT_RCLK, 2 },
{ 0x8086, 0x5ac0, 0xffff, 0, "Celeron N3350/Pentium N4200/Atom E3900 Series HSUART#2", 0x10,
        24 * DEFAULT_RCLK, 2 },
{ 0x8086, 0x5aee, 0xffff, 0, "Celeron N3350/Pentium N4200/Atom E3900 Series HSUART#3", 0x10,
        24 * DEFAULT_RCLK, 2 },

I can only verify that HSUART#2 works correctly, since the Fitlet hardware has only a single exposed RS-232 port (tested by connecting TX to RX, and observing the echo using kermit.).  There is a second physical port on an internal connector which is more difficult to get to.