ARM64: dts: marvell: armada-cp110: Add mg_core_clk for ethernet node

Marvell PPv2.2 controller present on CP-110 need the extra "mg_core_clk"
clock to avoid system hangs when powering some network interfaces up.

This issue appeared after a recent clock rework on Armada 7K/8K platforms.

This commit adds the new clock and updates the documentation accordingly.

[gregory.clement: use the real first commit to fix and add the cc:stable
flag]
Fixes: e3af9f7c ("RM64: dts: marvell: armada-cp110: Fix clock resources for various node")
Cc: <stable@vger.kernel.org>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>