pic: msm: Add support for ep config space access via sysfs

Perform ep config space dword read and write using 16-bit BDF,
offset and value to write as input.

To perform config space read for vendor and device id for BDF
0x200

echo 0x200 0x0 > config_dwrod_read

To perform config space write on status and control offset with
write value 0x400 for BDF 0x200

echo 0x200 0x4 0x400 > config_dword_write

Usage information is provided by reading config_dword_read and
config_dword_write.

Change-Id: Ia733b7102bb5961e7ab3b27d4438460e52a51c57
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>