net: freescale: select CONFIG_FIXED_PHY where needed

I ran into a randconfig build failure with CONFIG_FIXED_PHY=m
and CONFIG_GIANFAR=y:

x86_64-linux-ld: drivers/net/ethernet/freescale/gianfar.o:(.rodata+0x418): undefined reference to `fixed_phy_change_carrier'

It seems the same thing can happen with dpaa and ucc_geth, so change
all three to do an explicit 'select FIXED_PHY'.

The fixed-phy driver actually has an alternative stub function that
theoretically allows building network drivers when fixed-phy is
disabled, but I don't see how that would help here, as the drivers
presumably would not work then.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>