dmatest: run test via debugfs

Instead of doing
	modprobe dmatest ...
	modprobe -r dmatest
we allow user to run tests interactively.

The dmatest could be built as module or inside kernel. Let's consider those
cases.

1. When dmatest is built as a module...

After mounting debugfs and loading the module, the /sys/kernel/debug/dmatest
folder with nodes will be created. They are the same as module parameters with
addition of the 'run' node that controls run and stop phases of the test.

Note that in this case test will not run on load automatically.

Example of usage:
	% echo dma0chan0 > /sys/kernel/debug/dmatest/channel
	% echo 2000 > /sys/kernel/debug/dmatest/timeout
	% echo 1 > /sys/kernel/debug/dmatest/iterations
	% echo 1 > /sys/kernel/debug/dmatest/run

After a while you will start to get messages about current status or error like
in the original code.

Note that running a new test will stop any in progress test.

2. When built-in in the kernel...

The module parameters that is supplied to the kernel command line will be used
for the first performed test. After user gets a control, the test could be
interrupted or re-run with same or different parameters. For the details see
the above section "1. When dmatest is built as a module..."

In both cases the module parameters are used as initial values for the test case.
You always could check them at run-time by running
	% grep -H . /sys/module/dmatest/parameters/*

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>