Change in coreboot[master]: soc/mediatek: Add PCIe support

[Jianjun Wang (Code Review)]

Attention is currently required from: Hung-Te Lin, Arthur Heymans, Shelley Chen, Nico Huber, Furquan Shaikh, Paul Menzel, Angel Pons, Yu-Ping Wu.
Hello Shelley Chen, Hung-Te Lin, build bot (Jenkins), Yu-Ping Wu,

I'd like you to reexamine a change. Please visit

https://review.coreboot.org/c/coreboot/+/56791

to look at the new patch set (#12).

Change subject: soc/mediatek: Add PCIe support
......................................................................

soc/mediatek: Add PCIe support

Add PCIe support for MediaTek platform.

Reference:
  - MT8195 Register Map V0.3-2, Chapter 3.18 PCIe controller (Page 1250)
  - linux/drivers/pci/controller/pcie-mediatek-gen3.c

This code is based on MT8195 platform, but it should be common in each
platform with the same PCIe IP in the future.

TEST=Build pass and boot up to kernel successfully via SSD on Cherry
board, here is the SSD information in boot log:
 == NVME IDENTIFY CONTROLLER DATA ==
    PCI VID   : 0x1987
    PCI SSVID : 0x1987
    SN        : 28F40713077B0012602
    MN        : Phison ESE1A043-X28
    RAB       : 0x1
    AERL      : 0x3
    SQES      : 0x66
    CQES      : 0x44
    NN        : 0x1
Identified NVMe model Phison ESE1A043-X28

BUG=b:178565024

Signed-off-by: Jianjun Wang 
Change-Id: Ib9b6adaafa20aeee136372ec9564273f86776da0
---
M src/soc/mediatek/common/Kconfig
A src/soc/mediatek/common/include/soc/pcie_common.h
A src/soc/mediatek/common/pcie.c
3 files changed, 280 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/91/56791/12