arm64: dts: marvell: armada-37xx: Move PCIe max-link-speed property

Move the max-link-speed property of the PCIe node from board specific
device tree files to the generic armada-37xx.dtsi.

Armada 37xx supports only PCIe gen2 speed so max-link-speed property
should be in the generic armada-37xx.dtsi file.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Tomasz Maciej Nowak <tmn505@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>