Okay, the Evil Deed is done.  The sparc frontends use bus-space,
encapsulated in MD hooks for register access as suggested by CGD.
Thanks to David Brownlee for trying the patch.

I have code for the DEC ioctl asic code whic abuses the
register-access hooks, using them to handle device padding.  those
should really use a regmap.

I dont know how tight the timing constraints are on the sparc code,
or if it should even honour a regmap given the hw mappings, so I didnt
even try. If someone wants to add regmaps and decide what to do for
sparc (nothing?), that'd be great.