mei: expose fw version to sysfs

The ME FW version is constantly used by detection and update tools.
To improve the reliability and simplify these tools provide
a sysfs interface to access version of the platform ME firmware
in the following format:
<platform>:<major>.<minor>.<milestone>.<build>.
There can be up to three such blocks for different FW components.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>