Remove duplicate definitions in include/lxt971a.h.

Remove duplicate registers and bits definitions in
include/lxt971a.h for standard MII registers, and
use values in include/miiphy.h instead.
Signed-off-by: NHugo Villeneuve <hugo.villeneuve@lyrtech.com>
Signed-off-by: NBen Warren <biggerbadderben@gmail.com>