mii: include missing sources in loadable module

As of today, using 'kldload miibus' is not equivalent to using 'device
miibus' in a kernel config. Newly introduced PHY drivers (DP83822,
DP83867, VSCPHY) and source files/PHY driver for FDT-enabled kernels
are missing. Without including them, kernel modules using any function
from dev/mii/mii_fdt.c refuse to load. Additionally, miivar.h directly
includes opt_platform.h.

Add the missing sources to the module build, with the FDT-only files
gated behind an OPT_FDT check. Maintain the alphabetical listing of
SRCS, but move the required header files to a separate line to improve
readability.

Reviewed by: mhorne, mindal@semihalf.com
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D34256