net: phy: Move is_rgmii helper to phy.h

net: phy: Move is_rgmii helper to phy.h

Move the phy_interface_is_rgmii to the phy.h
file for all phy's to be able to use the API.

This now aligns with the Linux kernel based on
commit e463d88c36d42211aa72ed76d32fb8bf37820ef1

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Reviewed-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Tested-by: Mugunthan V N <mugunthanvnm@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>