net: phy: Add 5GBASER interface mode

Add 5GBASE-R phy interface mode

Signed-off-by: Pavana Sharma <pavana.sharma@digi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>