gpio: mt7621: support gpio-line-names property

This driver uses multiple gpiochip banks per device. To support
'gpio-line-names' along the banks 'offset' for each bank must be
set explicitly.

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