pinctrl: ab8505: Define group for GPIO pin 50

The Hulk robot reports an usused varible:

drivers/pinctrl/nomadik/pinctrl-ab8505.c:137:23: warning:
‘gpio50_a_1_pins’ defined but not used [-Wunused-const-variable=]
 static const unsigned gpio50_a_1_pins[] = { AB8505_PIN_L4 };
                       ^~~~~~~~~~~~~~~

This variable actually should be used. Probably an oversight
by the driver author.

Cc: Patrice Chotard <patrice.chotard@st.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Reported-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>