fsl/fman: allow modular build

ARM allmodconfig fails because of the addition of the FMAN driver:

drivers/built-in.o: In function `dtsec_restart_autoneg':
binder.c:(.text+0x173328): undefined reference to `mdiobus_read'
binder.c:(.text+0x173348): undefined reference to `mdiobus_write'
drivers/built-in.o: In function `dtsec_config':
binder.c:(.text+0x173d24): undefined reference to `of_phy_find_device'
drivers/built-in.o: In function `init_phy':
binder.c:(.text+0x1763b0): undefined reference to `of_phy_connect'
drivers/built-in.o: In function `stop':
binder.c:(.text+0x176014): undefined reference to `phy_stop'
drivers/built-in.o: In function `start':
binder.c:(.text+0x176078): undefined reference to `phy_start'

The reason is that the driver uses PHYLIB, but that is a loadable
module here, and fman itself is built-in.

This patch makes it possible to configure fman as a module as well
so we don't change the status of PHYLIB in an allmodconfig kernel,
and it adds a 'select PHYLIB' statement to ensure that phylib is
always built-in when fman is.

The driver uses "builtin_platform_driver(fman_driver);", which means
it cannot be unloaded, but it's still possible to have it as a loadable
module that gets loaded once and never removed.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 5adae51a ("fsl/fman: Add FMan MURAM support")
Signed-off-by: David S. Miller <davem@davemloft.net>