net: ethernet: ti: am65-cpsw-nuss: enable 10Mbps link speed in rgmii mode

In RGMII mode the 10Mbps link speed is supported only when CPSW2G MAC SL is
configured for External Control ("in band") mode
CPSW_PN_MAC_CONTROL_REG.CTL_EN(18) = 1

Hence update am65_cpsw_update_link() to follow documentation.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>