i2c: allow specifying separate wakeup interrupt in device tree

Instead of having each i2c driver individually parse device tree data in
case it or platform supports separate wakeup interrupt, and handle
enabling and disabling wakeup interrupts in their power management
routines, let's have i2c core do that for us.

Platforms wishing to specify separate wakeup interrupt for the device
should use named interrupt syntax in their DTSes:

	interrupt-parent = <&intc1>;
	interrupts = <5 0>, <6 0>;
	interrupt-names = "irq", "wakeup";

This patch is inspired by work done by Vignesh R <vigneshr@ti.com> for
pixcir_i2c_ts driver.

Note that the original code tried to preserve any existing wakeup
settings from userspace but was not quite right in that regard:
it would preserve wakeup flag set by userspace upon driver rebinding;
but it would re-arm the wakeup flag if it was disabled by userspace.

We think that resetting the flag upon re-binding the driver is proper
behavior as the driver is responsible for setting up and handling
wakeups.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Vignesh R <vigneshr@ti.com>
[wsa: updated the commit message]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>