ARM: dts: imx7d: cl-som-imx7: add compatible for phy

While not strictly needed as "ethernet-phy-ieee802.3-c22"
is assumed by default if not given explicitly, having
the compatible string here makes it more clear what
this is and which driver handles this - an Ethernet
phy attached to mdio, handled by of_mdio.c

Signed-off-by: André Draszik <git@andred.net>
CC: Ilya Ledvich <ilya@compulab.co.il>
CC: Igor Grinberg <grinberg@compulab.co.il>
CC: Rob Herring <robh+dt@kernel.org>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Shawn Guo <shawnguo@kernel.org>
CC: Sascha Hauer <s.hauer@pengutronix.de>
CC: Pengutronix Kernel Team <kernel@pengutronix.de>
CC: Fabio Estevam <festevam@gmail.com>
CC: NXP Linux Team <linux-imx@nxp.com>
CC: devicetree@vger.kernel.org
CC: linux-arm-kernel@lists.infradead.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>