Add a bus_add_child device method to bcm2835_sdhci.

This allows SDIO (through CAM) to attach to an upstream, e.g.,

..
sdhci_bcm0 pnpinfo name=mmc@7e300000 compat=brcm,bcm2835-mmc
  sdiob0
    ..

Without this, upon trying to load sdio, we would panic with
"bus_add_child is not implemented".

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation