genirq: implement read_irq_line for interrupt lines

Some drivers need to know what the status of the interrupt line is.
This is especially true for drivers that register a handler with
IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING and in the handler they
need to know which edge transition it was invoked for. Provide a way
for these handlers to read the logical status of the line after their
handler was invoked. If the line reads high it was called for a
rising edge and if the line reads low it was called for a falling edge.

The irq_read_line callback in the chip allows the controller to provide
the real time status of this line. Controllers that can read the status
of an interrupt line should implement this by doing necessary
hardware reads and return the logical state of the line.

Interrupt controllers based on the slow bus architecture should conduct
the transaction in this callback. The genirq code will call the chip's
bus lock prior to calling irq_read_line. Obviously since the transaction
would be completed before returning from irq_read_line it need not do
any transactions in the bus unlock call.

Change-Id: I3c8746706530bba14a373c671d22ee963b84dfab
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>
(cherry picked from commit ed3e47cb88b61859da3c221f22b509ebe0433218)

Conflicts:

	include/linux/interrupt.h