arm64: dts: imx8mp-debix-model-a: Use phy-mode

arm64: dts: imx8mp-debix-model-a: Use phy-mode

The property 'phy-connection-type' can be used to describe the interface
type between the Ethernet device and the Ethernet PHY device.

However, snps,dwmac.yaml gives the following warning:

imx8mp-debix-model-a.dtb: ethernet@30bf0000: 'phy-mode' is a required property
	from schema $id: http://devicetree.org/schemas/net/snps,dwmac.yaml#

To avoid the warning, switch to the more commonly used, 'phy-mode'
property instead.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>