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 <colin.king@canonical.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/gpio-stmpe.c

@@ -273,15 +273,21 @@ static void stmpe_dbg_show_one(struct seq_file *s,
 		u8 fall_reg;
 		u8 irqen_reg;
 
-		char *edge_det_values[] = {"edge-inactive",
+		static const char * const edge_det_values[] = {
+			"edge-inactive",
 			"edge-asserted",
-					   "not-supported"};
-		char *rise_values[] = {"no-rising-edge-detection",
+			"not-supported"
+		};
+		static const char * const rise_values[] = {
+			"no-rising-edge-detection",
 			"rising-edge-detection",
-				       "not-supported"};
-		char *fall_values[] = {"no-falling-edge-detection",
+			"not-supported"
+		};
+		static const char * const fall_values[] = {
+			"no-falling-edge-detection",
 			"falling-edge-detection",
-				       "not-supported"};
+			"not-supported"
+		};
 		#define NOT_SUPPORTED_IDX 2
 		u8 edge_det = NOT_SUPPORTED_IDX;
 		u8 rise = NOT_SUPPORTED_IDX;