Documentation: arm: marvell: Add link to public Armada 37xx Hardware Spec

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210121193418.22678-2-pali@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/arm/marvell.rst

@@ -185,6 +185,9 @@ EBU Armada family ARMv8
   Product Brief:
 	http://www.marvell.com/embedded-processors/assets/PB-88F3700-FNL.pdf
 
+  Hardware Spec:
+	http://www.marvell.com/content/dam/marvell/en/public-collateral/embedded-processors/marvell-embedded-processors-armada-37xx-hardware-specifications-2019-09.pdf
+
   Device tree files:
 	arch/arm64/boot/dts/marvell/armada-37*