arm64: defconfig: Set bcm2835-dma as built-in

With the introduction of 738987a1 ("mmc: bcm2835: Use
dma_request_chan() instead dma_request_slave_channel()") sdhost-bcm2835
now waits for its DMA channel to be available when defined in the
device-tree (it would previously default to PIO). Albeit the right
behaviour, the MMC host is needed for booting. So this makes sure the
DMA channel shows up in time.

Fixes: 738987a1 ("mmc: bcm2835: Use dma_request_chan() instead dma_request_slave_channel()")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>