diag: Add support for peripheral buffering modes

Diag on the peripherals has support to buffer diag data and drain
them as and when required. This patchset adds support on the apps
processor to enable different buffering modes on the peripherals.

Change-Id: I07b9ffe36dfc5c90ac0d8a8da91922e1127aed58
Signed-off-by: Ravi Aravamudhan <aravamud@codeaurora.org>