bcm4908: 5.15: fix compilation error for BCM4908 pinmux patch

bcm4908: 5.15: fix compilation error for BCM4908 pinmux patch

Fix compilation warning:
drivers/pinctrl/bcm/pinctrl-bcm4908.c: In function 'bcm4908_pinctrl_probe':
drivers/pinctrl/bcm/pinctrl-bcm4908.c:541:53: error: passing argument 3 of 'pinmux_generic_add_function' discards 'const' qualifier from pointer target type [-Werror=discarded-qualifiers]
  541 |                                             function->groups,
      |                                             ~~~~~~~~^~~~~~~~
In file included from drivers/pinctrl/bcm/pinctrl-bcm4908.c:16:
drivers/pinctrl/bcm/../pinmux.h:153:46: note: expected 'const char **' but argument is of type 'const char * const*'
  153 |                                 const char **groups,
      |                                 ~~~~~~~~~~~~~^~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>