wl1271: Add sysfs file to retrieve HW PG-version and ROM-version

This patch reads the HW PG version (along with a ROM-version, embedded in the
same value) from the wl1271 hardware and publishes the value in a sysfs -file.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>