net: fec: add stop mode support for imx8 platform

The current driver support stop mode by calling machine api.
The patch add dts support to set GPR register for stop request.

imx8mq enter stop/exit stop mode by setting GPR bit, which can
be accessed by A core.
imx8qm enter stop/exit stop mode by calling IMX_SC ipc APIs that
communicate with M core ipc service, and the M core set the related
GPR bit at last.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>