sdhci: add missing bus_add_child DEVMETHOD.

Add the missing bus_add_child DEVMETHOD. This is needed for the RPi5
running with a MMCCAM kernel and the worproject/rpi5-uefi to avoid a
kernel panic on boot when SDIO tries to attach to a 'Intel Bay Trail'
controller.

Reviewed by: imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D48152