arm64: dts: marvell: armada-37xx: add syscon compatible to NB clk node

Add "syscon" compatible to the North Bridge clocks node to allow the
cpufreq driver to access these registers via syscon API.

This is needed for a fix of cpufreq driver.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: e8d66e79 ("arm64: dts: marvell: armada-37xx: add nodes...")
Cc: stable@vger.kernel.org
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>