iwlwifi: extract iwl-csr.h

This patch extract CSR Register definition into separate
header files as most of the definition are commons to both
3945 and 4965.

Definitions specific for 3945 and 4965 are properly prefixed

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>