ARM: dts: at91: sama5d4 xplained: fix phy0 IRQ type

For phy0 KSZ8081, the type of GPIO IRQ should be "level low" instead of
"edge falling".

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Fixes: 38153a01 ("ARM: at91/dt: sama5d4: add dts for sama5d4 xplained board")
Cc: <stable@vger.kernel.org> # 4.1+