mfd: arizona: Avoid use of legacy IRQ mapping

regmap_add_irq_chip is called from arizona_irq_init with the irq_base
specified as -1 and regmap_add_irq_chip uses if (irq_base) to check if
it should use legacy IRQ mapping. As such the irq mappings are currently
added with irq_domain_add_legacy, rather than irq_domain_add_linear.
This is clearly a typo as there is no reason why this driver can't use
irq_domain_add_linear.

This patch corrects this by passing the irq_base as zero to
regmap_add_irq_chip.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>