CSR ordering support in generated files

[AndrewD]

Add CSR_ORDERING constants and update access functions. Unchanged behaviour with default csr_ordering="big".

Can use --csr-ordering little on command line or parser.set_defaults(csr_ordering="little").

Note that this only affects the addressing of CSR that are 64bit or wider.

This is a proof of concept implementation - only confirmed by inspection so far.