arm64: dts: remove broken DTSI links

Both bcm283x-rpi-csi1-2lane.dtsi and bcm283x-rpi-lan7515.dtsi are linked to
"../arm/boot/dts/" instead of "../arm/boot/dts/broadcom", so the links are
broken.

Fixes: d060fc0b ("BCM2708: Add core Device Tree support")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>