arm: switch the BUSDMA buffers to normal uncached memory

The BUSDMA buffer is treated as normal memory during compilation and compiler
is free to inline/optimize basic functions (i.e. memset, memcpy) accessing
buffers, including when an instruction is generated that performs a word
access to unaligned data. We support this, but only if the buffer in question
is mapped as normal memory (cached or not), but not to memory mapped as
strongly ordered or device type.

MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D47485