irqchip/renesas-irqc: Add helper variable dev = &pdev->dev

The probe function uses "&pdev->dev" a lot, hence add a shorthand for
that.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>