irqchip/armada-370-xp: Do not touch IPI registers on platforms without IPI

On platforms where IPI is not available in the MPIC, the IPI registers
instead represent an additional set of MSI interrupt registers (currently
unused by the driver).

Do not touch these registers on platforms where IPI is not available in the
MPIC.

[ Marek: refactored, 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>