pinctrl: st: Add Interrupt support

This patch add interrupt support to the pincontroller driver.

ST Pincontroller GPIO bank can have one of the two possible types of
interrupt-wirings.

First type is via irqmux, single interrupt is used by multiple gpio
banks. This reduces number of overall interrupts numbers required. All
these banks belong to a single pincontroller.
		  _________
		 |	   |----> [gpio-bank (n)    ]
		 |	   |----> [gpio-bank (n + 1)]
	[irqN]-- | irq-mux |----> [gpio-bank (n + 2)]
		 |	   |----> [gpio-bank (...  )]
		 |_________|----> [gpio-bank (n + 7)]

Second type has a dedicated interrupt per gpio bank.

	[irqN]----> [gpio-bank (n)]

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>