net: phy: Add Meson GXL PHY hardware dependency

As I understand it the Meson GXL PHY driver is only useful on one
architecture so only make it visible on that architecture.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: 7334b3e4 ("net: phy: Add Meson GXL Internal PHY driver")
Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>