dt-bindings: net: dwmac: fix 'mac-mode' type

The 'mac-mode' property is similar to 'phy-mode' and 'phy-connection-type',
which are enums of mode strings.

The 'dwmac' driver supports almost all modes declared in the 'phy-mode'
enum (except for 1 or 2). But in general, there may be a case where
'mac-mode' becomes more generic and is moved as part of phylib or netdev.

In any case, the 'mac-mode' field should be made an enum, and it also makes
sense to just reference the 'phy-connection-type' from
'ethernet-controller.yaml'. That will also make it more future-proof for new
modes.

Fixes: 9c15d359

 ("dt-bindings: net: dwmac: document 'mac-mode' property")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>