i.-MX6 eth driver eth_plat.h should not include src/plat/imx6/enet.h

[axel-h]

Follow-up from https://github.com/seL4/util_libs/pull/112/files/b6a47d9d0b656ef77601ea9d005a26409c887a8c#r766004484

We had to include this header file here to address a nasty include problem and make these prototypes available to other C files in our driver:

int enet_mdio_read(struct enet *enet, uint16_t phy, uint16_t reg);
int enet_mdio_write(struct enet *enet, uint16_t phy, uint16_t reg, uint16_t data);

The includes should be reworked, so no header form the src are made public