FIXED_PHY must depend on PHYLIB=y

This patch fixes the following build error introduced by commit
a79d8e93

 and reported by Olaf Hering:

<--  snip  -->

...
  LD      .tmp_vmlinux1
arch/powerpc/sysdev/built-in.o: In function `of_add_fixed_phys':
fsl_soc.c:(.init.text+0xd34): undefined reference to `fixed_phy_add'
make: *** [.tmp_vmlinux1] Error 1

<--   snip  -->

Signed-off-by: Adrian Bunk <adrian.bunk@movial.fi>
Signed-off-by: Jeff Garzik <jeff@garzik.org>