ARM: bcm283x: Allocate all mailbox buffers cacheline aligned

The mailbox buffer is required to be at least 16 bytes aligned, but for
cache invalidation and/or flush it needs to be cacheline aligned.
Use ALLOC_CACHE_ALIGN_BUFFER for all mailbox buffer allocations.

Signed-off-by: Alexander Stein <alexanders83@web.de>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>