pinctrl: nomadik: abx500: constify gpio_chip structure

This structure is only used to copy into another structure, so declare
it as const.

This issue was detected using Coccinelle and the following semantic patch:

@r disable optional_qualifier@
identifier i;
position p;
@@
static struct gpio_chip i@p = { ... };

@ok@
identifier r.i;
expression e;
position p;
@@
e = i@p;

@bad@
position p != {r.p,ok.p};
identifier r.i;
struct gpio_chip e;
@@
e@i@p

@depends on !bad disable optional_qualifier@
identifier r.i;
@@
static
+const
 struct gpio_chip i = { ... };

In the following log you can see a significant difference in the code size
and data segment, hence in the dec segment. This log is the output
of the size command, before and after the code change:

before:
   text    data     bss     dec     hex filename
  17545    5376       0   22921    5989 drivers/pinctrl/nomadik/pinctrl-abx500.o

after:
            bss     dec     hex filename
  17273    5320       0   22593    5841 drivers/pinctrl/nomadik/pinctrl-abx500.o
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/nomadik/pinctrl-abx500.c

@@ -647,7 +647,7 @@ static inline void abx500_gpio_dbg_show_one(struct seq_file *s,
 #define abx500_gpio_dbg_show	NULL
 #endif
 
-static struct gpio_chip abx500gpio_chip = {
+static const struct gpio_chip abx500gpio_chip = {
 	.label			= "abx500-gpio",
 	.owner			= THIS_MODULE,
 	.request		= gpiochip_generic_request,