ath79: ar7100: remove IRQ code from PCI driver

ath79: ar7100: remove IRQ code from PCI driver

Currently all PCI devices get the same IRQ that affects performance badly.

This commit adresses this problem and cleans the code.

ar7100 has a special PCI interrupt controller@18060018 that works exactly
the same way as misc interrupt controller.

This patch does the following:

1. Removes all IRQ handling code from the PCI driver.
2. Defines pci-intc interrupt controller@18060018 in dtsi.
3. Removes interrupt-controller property from PCI node.
4. Sets a correct interrupt mask for PCI devices.

Run tested on DIR-825 B1.

Signed-off-by: Dmitry Tunin <hanipouspilot@gmail.com>