pci_dw: Support modern "unroll" iATU mode

[jrtc27]

Summary

This supersedes the old legacy mode where a viewport register was used
to mux multiple regions behind a single set of registers, and is used on
the SiFive FU740.

[mmel]

On correctly implemented systems, reading from an unimplemented device register will cause an exception (external asynchronous interrupt on arm64), on real systems it will return an arbitrary value.
Therefore, pci_dw_detect_atu_unroll() should only be called for modern core versions (which have the DW_IATU_VIEWPORT register implemented) - so we should check the core version first.
Otherwise looks good to me.

[mmel]

Please, ignore my previous comment - I overlooked register name :(

Tested on MACCHIATObin and HoneyComb LX2.