irqchip: crossbar: Skip some irqs from getting mapped to crossbar

When, in the system due to varied reasons, interrupts might be unusable
due to hardware behavior, but register maps do exist, then those interrupts
should be skipped while mapping irq to crossbars.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
irqchip: crossbar: Initialise the crossbar with a safe value

Since crossbar is s/w configurable, the initial settings of the
crossbar cannot be assumed to be sane. This implies that:
a) On initialization all un-reserved crossbars must be initialized to
   a known 'safe' value.
b) When unmapping the interrupt, the safe value must be written to
   ensure that the crossbar mapping matches with interrupt controller
   usage.

So provide a safe value in the compatible data to map if
'0' is not safe for the platform and use it during init and unmap

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
irqchip: crossbar: create havoc by reversing search for free irqs

Reverse the search algorithm to ensure that address mapping and IRQ
allocation logics are proper. This opens up a whole new world of bugs
that is easily fixable rather than wait till allocation logic approaches
the limit to find new bugs.

Signed-off-by: Nishanth Menon <nm@ti.com>