regulator: Constify constraints name

There's no need for the API to modify it and having it const makes it
easier to use with random strings the board code has.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/regulator/machine.h

@@ -95,7 +95,7 @@ struct regulator_state {
  */
 struct regulation_constraints {
 
-	char *name;
+	const char *name;
 
 	/* voltage output range (inclusive) - for voltage control */
 	int min_uV;