gpio: gpio-stmpe: make various char arrays static const, shrinks object size

Don't populate the read-only arrays edge_det_values, rise_values and
fall_values on the stack but instead make them static and constify them.
Makes the object code smaller by over 240 bytes:

Before:
text data bss dec hex filename
9525 2520 192 12237 2fcd drivers/gpio/gpio-stmpe.o

After:
text data bss dec hex filename
9025 2776 192 11993 2ed9 drivers/gpio/gpio-stmpe.o

(gcc version 7.2.0 x86_64)

Signed-off-by: Colin Ian King <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-stmpe.c

···
 		u8 fall_reg;
 		u8 irqen_reg;
 
-		char *edge_det_values[] = {"edge-inactive",
-					   "edge-asserted",
-					   "not-supported"};
-		char *rise_values[] = {"no-rising-edge-detection",
-				       "rising-edge-detection",
-				       "not-supported"};
-		char *fall_values[] = {"no-falling-edge-detection",
-				       "falling-edge-detection",
-				       "not-supported"};
+		static const char * const edge_det_values[] = {
+			"edge-inactive",
+			"edge-asserted",
+			"not-supported"
+		};
+		static const char * const rise_values[] = {
+			"no-rising-edge-detection",
+			"rising-edge-detection",
+			"not-supported"
+		};
+		static const char * const fall_values[] = {
+			"no-falling-edge-detection",
+			"falling-edge-detection",
+			"not-supported"
+		};
 		#define NOT_SUPPORTED_IDX 2
 		u8 edge_det = NOT_SUPPORTED_IDX;
 		u8 rise = NOT_SUPPORTED_IDX;