net: phy: bcm63xx: Allow to be built with COMPILE_TEST

There is nothing in this driver which prevents it to be compiled for
other architectures. Add COMPILE_TEST so we get better compile test
coverage.

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