term/serial: Add support for PCI serial devices

term/serial: Add support for PCI serial devices

Loosely based on early_pci_serial_init() from Linux, allow GRUB to make
use of PCI serial devices.

Specifically, my Alderlake NUC exposes the Intel AMT SoL UART as a PCI
enumerated device but doesn't include it in the EFI tables.

Tested and confirmed working on a "Lenovo P360 Tiny" with Intel AMT
enabled. This specific machine has (from lspci -vv):

00:16.3 Serial controller: Intel Corporation Device 7aeb (rev 11) (prog-if 02 [16550])
        DeviceName: Onboard - Other
        Subsystem: Lenovo Device 330e
        Control: I/O+ Mem+ BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
        Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
        Interrupt: pin D routed to IRQ 19
        Region 0: I/O ports at 40a0 [size=8]
        Region 1: Memory at b4224000 (32-bit, non-prefetchable) [size=4K]
        Capabilities: [40] MSI: Enable- Count=1/1 Maskable- 64bit+
                Address: 0000000000000000  Data: 0000
        Capabilities: [50] Power Management version 3
                Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
        Kernel driver in use: serial

From which the following config (/etc/default/grub) gets a working
serial setup:

GRUB_CMDLINE_LINUX="console=tty0 earlyprintk=pciserial,00:16.3,115200 console=ttyS0,115200"
GRUB_SERIAL_COMMAND="serial --port=0x40a0 --speed=115200"
GRUB_TERMINAL="serial console"

Documentation is added to note that serial devices found on the PCI bus will
be exposed as "pci,XX:XX.X" and how to find serial terminal logical names.
Also, some minor documentation improvements were added.

This can be tested in QEMU by adding a pci-serial device, e.g. using the option
"-device pci-serial".

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>