wil6210: add debugfs to show PMC ring content

PMC is a hardware debug mechanism which allows capturing real time
debug data and stream it to host memory. The driver allocates memory
buffers and set them inside PMC ring of descriptors.
Add pmcring debugfs that application can use to read the binary
content of descriptors inside the PMC ring (cat pmcring).

Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>