gpio: put GPIO IRQs into their own lock class

Another feature that is duplicated in a number of GPIO irqchips
is that these cascades IRQs are assigned their own lock class
so as to avoid warnings about lockdep recursions. Do this also
in the generic GPIO irqchip helpers for smooth transition to
this core infrastructure.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>