drivers: gpio_sifive: Fixup old DT_INST macro

Convert DT_INST_0_SIFIVE_GPIO0_IRQ_##n to use the new DT macros:
DT_INST_IRQ_BY_IDX(0, n, irq).  Also tweak a use of
DT_INST_IRQ_HAS_CELL(0, irq) to DT_INST_IRQ_HAS_IDX(0, 0) to match
style.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>