mvneta: add FIXED_PHY dependency

mvneta: add FIXED_PHY dependency

The fixed_phy infrastructure is done in a way that is optional,
by providing 'static inline' helper functions doing nothing in
include/linux/phy_fixed.h for all its APIs. However, three out
of the four users (DSA, BCMGENET, and SYSTEMPORT) always
'select FIXED_PHY', presumably because they need that.
MVNETA is the fourth one, and if that is built-in but FIXED_PHY
is configured as a loadable module, we get a link error:

drivers/built-in.o: In function `mvneta_fixed_link_update':
fpga-mgr.c:(.text+0x33ed80): undefined reference to `fixed_phy_update_state'

Presumably this driver has the same dependency as the others,
so this patch also uses 'select' to ensure that the fixed-phy
support is built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 898b2970e2c9 ("mvneta: implement SGMII-based in-band link state signaling")
Signed-off-by: David S. Miller <davem@davemloft.net>