PCI: sg2042: Add Sophgo SG2042 PCIe driver

PCI: sg2042: Add Sophgo SG2042 PCIe driver

Add support for PCIe controller in Sophgo SG2042 SoC. The controller uses
the Cadence PCIe core programmed by pcie-cadence* common driver. The PCIe
controller in SG2042 works in host mode only, supporting data rate up to 16
GT/s and lanes up to x16 or x8.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
[mani: reworded description and minor code cleanups]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/01b0a57cd9dba8bed7c1f2d52997046c2c6f042b.1757643388.git.unicorn_wang@outlook.com