gpio: xlp: Fix vulcan IRQ descriptor allocation

irq_alloc_descs need not be called in case of Vulcan, where we use
a dynamic IRQ range for GPIO interrupt numbers.

Update code not to call irq_alloc_descs and pass 0 as irq_base in
case of Vulcan.

Signed-off-by: Kamlakant Patel <kamlakant.patel@broadcom.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>