net: ethernet: ti: cpsw: add check for in-band mode setting with RGMII PHY interface

According to AM335x TRM[1] 14.3.6.2, AM437x TRM[2] 15.3.6.2 and
DRA7 TRM[3] 24.11.4.8.7.3.3, in-band mode in EXT_EN(bit18) register is only
available when PHY is configured in RGMII mode with 10Mbps speed. It will
cause some networking issues without RGMII mode, such as carrier sense
errors and low throughput. TI also mentioned this issue in their forum[4].

This patch adds the check mechanism for PHY interface with RGMII interface
type, the in-band mode can only be set in RGMII mode with 10Mbps speed.

References:
[1]: https://www.ti.com/lit/ug/spruh73p/spruh73p.pdf
[2]: http://www.ti.com/lit/ug/spruhl7h/spruhl7h.pdf
[3]: http://www.ti.com/lit/ug/spruic2b/spruic2b.pdf
[4]: https://e2e.ti.com/support/arm/sitara_arm/f/791/p/640765/2392155



Suggested-by: Holsety Chen (陳憲輝) <Holsety.Chen@moxa.com>
Signed-off-by: SZ Lin (林上智) <sz.lin@moxa.com>
Signed-off-by: Schuyler Patton <spatton@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>