irqchip/armada-370-xp: Do not allow mapping IRQ 0 and 1

IRQs 0 (IPI) and 1 (MSI) are handled internally by this driver,
generic_handle_domain_irq() is never called for these IRQs.

Disallow mapping these IRQs.

[ Marek: changed commit message ]

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>