Yes, but syscon_regmap_lookup_by_phandle() doesn't need any support for
additional parameters. Have a look at:

drivers/net/ethernet/stmicro/stmmac/dwmac-socfpga.c

First get the regmap, then the 1st argument is the offset in the regmap,
the 2nd and 3rd could be the bits.

So, for one driver the extra arguments are: <reg> <start_bit> <stop_bit>
For another driver (the stmmac example): <reg_offset> <reg_shift>

Phew... Then we should really have a "syscon-raminit" property probably,
so that at least plain "syscon" has a consistent syntax?

quoted

So, I'd rather drop additional arguments.

Why would you like to have it encoded in DT?

Where put the information then? Into the driver, but where do you get
the reference which instance of the DCAN you are, so that you can look
up the correct bits?

Agreed. I thought we had this information in the driver already, but we
haven't...