> Should we add match data to this compatible string to identify the variant
> and explicitly reject probe with -EOPNOTSUPP if the clock configuration
> would require the unsupported SUBU mode?

Correct. Matches also my notes which say that this code

445                 if (rate != 32768)
446                         scmp_val = RZN1_RTC_CTL0_SLSB_SCMP;

is wrong when we have a variant which has no SUBU support. Because in
that case, we need to enable SCMP even for 32768Hz.

So, yes, it is needed to mark which variant has SUBU (RZ/N1 only, likely
forever) and bail out if no SUBU and no 'xtal'. The above code needs to
check SUBU support, too.