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>