pinctrl: qcom: ssbi-gpio: constify gpio_chip structure

This structure is only used to copy into other 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
  17061    6992       0   24053    5df5 drivers/pinctrl/qcom/pinctrl-ssbi-gpio.o

after:
   text    data     bss     dec     hex filename
  16777    6904       0   23681    5c81 drivers/pinctrl/qcom/pinctrl-ssbi-gpio.o
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/qcom/pinctrl-ssbi-gpio.c

@@ -588,7 +588,7 @@ static void pm8xxx_gpio_dbg_show(struct seq_file *s, struct gpio_chip *chip)
 #define pm8xxx_gpio_dbg_show NULL
 #endif
 
-static struct gpio_chip pm8xxx_gpio_template = {
+static const struct gpio_chip pm8xxx_gpio_template = {
 	.direction_input = pm8xxx_gpio_direction_input,
 	.direction_output = pm8xxx_gpio_direction_output,
 	.get = pm8xxx_gpio_get,