Instead of allocating descriptors on-the-fly for the device tree
initialization case, use irq_domain_add_simple() which will take
care of this if you pass negative as the first_irq.

Alter the signature of __vic_init() to pass the first_irq as
signed so this works as expected.

Switching the VIC to use irq_domain_add_simple() also has the
upside of displaying the same WARNING when you boot with
pre-allocated descriptors on systems using SPARSE_IRQ but
yet not using device tree.

Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>