ARM: shmobile: r8a7794: alt: Fix ethernet controller PHY IRQ line

The PHY IRQ line is connected to external IRQ8, not IRQ0. Fix DT
accordingly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>