bcm_dma: attach at an earlier bus pass

The sdhci_bcm driver attach routine relies on bcm_dma already being
attached, in order to allocate a DMA channel. However, both drivers
attached at the default pass so this is not guaranteed. Newer RPI
firmware exposes this assumption, and the result is a NULL-dereference
in bcm_dma_allocate().

To fix this, use BUS_PASS_SUPPORTDEV for bcm_dma.

PR: 268835
Reviewed by: mhorne
MFC after: 1 week