pinctrl: qcom: ssbi: mpp: 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
  15136    5112       0   20248    4f18 drivers/pinctrl/qcom/pinctrl-ssbi-mpp.o

after:
            bss     dec     hex filename
  14849    5024       0   19873    4da1 drivers/pinctrl/qcom/pinctrl-ssbi-mpp.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-mpp.c

@@ -643,7 +643,7 @@ static void pm8xxx_mpp_dbg_show(struct seq_file *s, struct gpio_chip *chip)
 #define pm8xxx_mpp_dbg_show NULL
 #endif
 
-static struct gpio_chip pm8xxx_mpp_template = {
+static const struct gpio_chip pm8xxx_mpp_template = {
 	.direction_input = pm8xxx_mpp_direction_input,
 	.direction_output = pm8xxx_mpp_direction_output,
 	.get = pm8xxx_mpp_get,