irqchip: add basic infrastructure

From: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>

With the recent creation of the drivers/irqchip/ directory, it is
desirable to move irq controller drivers here. At the moment, the only
driver here is irq-bcm2835, the driver for the irq controller found in
the ARM BCM2835 SoC, present in Rasberry Pi systems. This irq
controller driver was exporting its initialization function and its
irq handling function through a header file in
<linux/irqchip/bcm2835.h>.

When proposing to also move another irq controller driver in
drivers/irqchip, Rob Herring raised the very valid point that moving
things to drivers/irqchip was good in order to remove more stuff from
arch/arm, but if it means adding gazillions of headers files in
include/linux/irqchip/, it would not be very nice.

So, upon the suggestion of Rob Herring and Arnd Bergmann, this commit
introduces a small infrastructure that defines a central
irqchip_init() function in drivers/irqchip/irqchip.c, which is meant
to be called as the ->init_irq() callback of ARM platforms. This
function calls of_irq_init() with an array that will progressively
contain the compatible strings of each irq controller driver, and also
a reference to the initialization functions of such drivers. The
drivers/irqchip/irqchip.h header file, currently empty, is added to
allow irq controller drivers to expose their initialization function
to the main irqchip.c file. Note that the irq controller driver
initialization function is responsible for setting the global
handle_arch_irq() variable, so that ARM platforms no longer have to
define the ->handle_irq field in their DT_MACHINE structure.

A global header, <linux/irqchip.h> is also added to expose the single
irqchip_init() function to the reset of the kernel.

A further commit moves the BCM2835 irq controller driver to this new
small infrastructure, therefore removing the include/linux/irqchip/
directory.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
Reviewed-by: Stephen Warren <swarren at wwwdotorg.org>
Reviewed-by: Rob Herring <rob.herring at calxeda.com>