net: ipa: implement MAX_READS_BEATS QSB data

Starting with IPA v4.0, a limit is placed on the number of bytes
outstanding in a transaction, to reduce latency.  The limit is
imposed only if this value is non-zero.

We don't use a non-zero value for SC7180, but newer versions of IPA
do.  Prepare for that by allowing a programmed value to be specified
in the platform configuration data.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>