dt-bindings: gpio: altera: Fix altr,interrupt-type property

The altr,interrupt-trigger property is not used by the driver.
Instead, altr,interrupt-type is used by the driver and the driver
does not probe if this property is not specified. Therefore, it
is expected that there are no users of the -trigger property in
the wild and that this is a typo in the documentation for the
altera-pio controller. This patch fixes the typo.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tien Hock Loh <thloh@altera.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>