DMA heap may be placed above 1GB

[thebel1]

This will cause the VideoCore to be unable to DMA to the mailbox buffer.

A few options:
a) Ensure the driver is loaded before others that require a sub-2GB heap
b) Make a test allocation on the DMA heap and fail to attach to the ACPI device if the MA is above 1GB
c) Program the DMA controller to move the aperture up to match the start of the first heap segment