ARM64/SND_BCM2835: Port it to arm64.

ARM64/SND_BCM2835: Port it to arm64.

In the messages sent to VCHIQ, SND_BCM2835 passes a callback
and a context into two 32 bit pointers.  Since this
message is interpreted by the firmware, it can't be easily
changed.  Luckily only one of these fields is actual used,
so on ARM64 only use one of the fields to store the upper
half of the 64 bit callback pointer.

The Kconfig is also changed to allow ARM audio to work for ARM64
as well.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>