gpio: brcmstb: remove custom 'brcmstb_gpio_set_names'

Gpiolib core code has been updated to support setting friendly names
through properly 'gpio-line-names'. Instead of redefine behaviour here
to skip the core to be executed, just properly assign the desired offset
per bank to get in the core the expected behaviour.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>