irqchip/armada-370-xp: Add support for 32 MSI interrupts on non-IPI platforms

The doorbell interrupts have the following layout on IPI vs no-IPI
platforms:

                    |  0...7  |  8...15  |       16...31       |
  ------------------+---------+----------+---------------------+
       IPI platform |   IPI   |   n/a    |         MSI         |
  ------------------+---------+----------+---------------------+
   non-IPI platform |                   MSI                    |
  ------------------+------------------------------------------+

Currently the driver only allows for the upper 16...31 interrupts for
MSI domain (i.e. the MSI domain has only 16 interrupts).

On platforms where IPI is not available, we can use whole 32 MSI
interrupts.

Implement support also for the lower 16 MSI interrupts on non-IPI
platforms.

[ 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>