net: stmmac: dwmac4: Allow platforms to specify some DMA/MTL offsets

Some platforms have dwmac4 implementations that have a different
address space layout than the default, resulting in the need to define
their own DMA/MTL offsets.

Extend the functions to allow a platform driver to indicate what its
addresses are, overriding the defaults.

Signed-off-by: Andrew Halaney <ahalaney@redhat.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Brian Masney <bmasney@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>