mfd: constify regmap_irq_chip structures

Declare regmap_irq_chip structures as const as they are only stored
in the regmap_irq_chip field of a rk808 structure. This field is of
type const, so regmap_irq_chip structures having this property can be
made const too.
Done using Coccinelle:

@r disable optional_qualifier@
identifier x;
position p;
@@
static struct regmap_irq_chip x@p={...};

@ok@
struct rk808 a;
identifier r.x;
position p;
@@
a.regmap_irq_chip=&x@p;

@bad@
position p != {r.p,ok.p};
identifier r.x;
@@
x@p

@depends on !bad disable optional_qualifier@
identifier r.x;
@@
+const
struct regmap_irq_chip x;

File size before:
   text	   data	    bss	    dec	    hex	filename
   5033	    584	     16	   5633	   1601	drivers/mfd/rk808.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   5225	    392	     16	   5633	   1601	drivers/mfd/rk808.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>