staging: bcm2835-audio: Add missing MODULE_ALIAS

Commit 8353fe6f ("Revert "staging: bcm2835-audio: Drop DT
dependency"") reverts the upstream change and makes bcm2835-audio use
device tree again, however, it also removes the MODULE_ALIAS for the
platform device. This MODULE_ALIAS is needed, because VCHIQ registers
bcm2835-audio as a child platform device since commit 25c7597a
("staging: vchiq_arm: Register a platform device for audio"), and this
mechanism is adopted also in the downstream kernel.

This commit puts back that MODULE_ALIAS to make bcm2835-audio
autoprobing work again. The rest of VCHIQ children have their
MODULE_ALIASes in place.

Fixes: 8353fe6f ("Revert "staging: bcm2835-audio: Drop DT dependency"")
Signed-off-by: Maxim Mikityanskiy <maxtram95@gmail.com>