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

sys/dev/sdhci/sdhci_acpi.c

@@ -420,6 +420,7 @@ static device_method_t sdhci_methods[] = {
 	/* Bus interface */
 	DEVMETHOD(bus_read_ivar,	sdhci_generic_read_ivar),
 	DEVMETHOD(bus_write_ivar,	sdhci_generic_write_ivar),
+	DEVMETHOD(bus_add_child,	bus_generic_add_child),
 
 	/* mmcbr_if */
 	DEVMETHOD(mmcbr_update_ios,	sdhci_generic_update_ios),