mfd: vexpress-sysreg: Switch to gpiochip_add_data()

We're planning to remove the gpiochip_add() function to swith
to gpiochip_add_data() with NULL for data argument.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>