net: phy: Add Meson GXL Internal PHY driver

Add driver for the Internal RMII PHY found in the Amlogic Meson GXL SoCs.

This PHY seems to only implement some standard registers and need some
workarounds to provide autoneg values from vendor registers.

Some magic values are currently used to configure the PHY, and this a
temporary setup until clarification about these registers names and
registers fields are provided by Amlogic.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>