irqchip: Return -EPERM for reserved IRQs

The irqdomain core will report a log message for any attempted map call
that fails unless the error code is -EPERM. This patch changes the
Versatile irq controller drivers to use -EPERM because it is normal for
a subset of the IRQ inputs to be marked as reserved on the various
Versatile platforms.

Signed-off-by: Grant Likely <grant.likely@linaro.org>