gpio: mvebu: use platform_irq_count

platform_irq_count() is the more generic way (independent of
device trees) to determine the count of available interrupts. So
use this instead.

As platform_irq_count() might return an error code (which
of_irq_count doesn't) some additional handling is necessary.

Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>