mei: separate compilation of the ME hardware specifics

We add struct mei_hw_ops to virtualize access to hw specific
configurations. This allows us to separate the compilation
of the ME interface from the ME hardware specifics

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