mfd: Allow for const stmpe keyboard data

Since it's not like we will re-arrange the keys at run-time, it
seems proper to allow the keymap data to be const. This solves
a compilation warning in ux500.

Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

include/linux/mfd/stmpe.h

@@ -117,7 +117,7 @@ struct matrix_keymap_data;
  * @no_autorepeat: disable key autorepeat
  */
 struct stmpe_keypad_platform_data {
-	struct matrix_keymap_data *keymap_data;
+	const struct matrix_keymap_data *keymap_data;
 	unsigned int debounce_ms;
 	unsigned int scan_count;
 	bool no_autorepeat;