Looks like there are actually already two different drivers for some kind of sd@spi, but they live under evbsh3 -- so, not sure if they're actually specific to sh3 boards or what:
https://nxr.netbsd.org/xref/src/sys/arch/evbsh3/t_sh7706lan/scimci.c https://nxr.netbsd.org/xref/src/sys/arch/evbsh3/t_sh7706lan/ssumci.c Maybe nonaka@ knows more?
