gpio: sifive: To get gpio irq offset from device tree data

We can get hwirq number of the gpio by its irq_data->hwirq so that we don't
need to add more macros for different platforms. This patch is tested in
SiFive Unleashed board and SiFive Unmatched board.

Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>