drivers: dw-axi-dmac: make more sensible choices about memory accesses

There's no real need to constrain MEM access widths to 32-bit (or
narrower), as the DMAC is intelligent enough to size memory accesses
appropriately. Wider accesses are more efficient.

Similarly, MEM burst lengths don't need to be a function of DEV burst
lengths - the DMAC packs/unpacks data into/from its internal channel
FIFOs appropriately. Longer accesses are more efficient.

However, the DMAC doesn't have complete support for unaligned accesses,
and blocks are always defined in integer multiples of SRC_WIDTH, so odd
source lengths or buffer alignments will prevent wide accesses being
used, as before.

There is an implicit requirement to limit requested DEV read burst
lengths to less than the hardware's maximum configured MSIZE - otherwise
RX data will be left over at the end of a block. There is no config
register that reports this value, so the AXI burst length parameter is
used to produce a facsimile of it. Warn if such a request arrives that
doesn't respect this.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>