soc_core/cpu: add io_regions and deprecate shadow_base (with API retro-compat)

soc_core/cpu: add io_regions and deprecate shadow_base (with API retro-compat)

The shadow_base parameter has always been difficult to apprehend, replace it with
io_regions (uncached regions) defined user or the CPU.

The equivalent of a shadow_base parameter of 0x80000000 in the old API is:
io_regions = {0x80000000: 0x80000000} # origin, length

It's still possible to use shadow_base with retro-compat, but user is encouraged
to update and features will be removed in the future.