Add a keymatrix_t type

This contains both the matrix number and key position, in preparation
for multi-matrix support

keyboards/planck/old_keymap_files/keymap_common.c

@@ -18,7 +18,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 
 /* translates key to keycode */
-uint8_t keymap_key_to_keycode(uint8_t layer, keypos_t key)
+uint8_t keymap_key_to_keycode(uint8_t layer, keymatrix_t key)
 {
     return pgm_read_byte(&keymaps[(layer)][(key.row)][(key.col)]);
 }