dt-bindings: fec: update the gpr property

- rename the 'gpr' property string to 'fsl,stop-mode'.
- Update the property to define gpr register offset and
bit in DT, since different instance have different gpr bit.

v2:
 * rename 'gpr' property string to 'fsl,stop-mode'.

Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>