phy layer: add kernel-doc + DocBook

Convert function documentation in drivers/net/phy/ to kernel-doc
and add it to DocBook.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>