microblaze/irqchip: Move intc driver to irqchip

The Xilinx AXI Interrupt Controller IP block is used by the MIPS
based xilfpga platform and a few PowerPC based platforms.

Move the interrupt controller code out of arch/microblaze so that
it can be used by everyone

Tested-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>