fdt: Move fdt_fixup_ethernet to a common place

With 3f66149d we no longer have a common call fdt_fixup_ethernet.
This was fine to do on PowerPC as they largely had calls already in
ft_cpu_fixup.  On ARM however we largely relied on this call.  Rather
than introduce a large number of changes to ft_cpu_fixup /
ft_board_fixup we recognize that this is a common enough call that we
should be doing it in a central location.  Do it early enough that we
can do any further updates in ft_cpu_fixup / ft_board_fixup.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Thomas Chou <thomas@wytron.com.tw> (maintainer:NIOS)
Cc: York Sun <york.sun@nxp.com> (maintainer:POWERPC MPC85XX)
Cc: Stefan Roese <sr@denx.de> (maintainer:POWERPC PPC4XX)
Cc: Simon Glass <sjg@chromium.org>
Cc: Joakim Tjernlund <Joakim.Tjernlund@infinera.com>
Fixes: 3f66149d ("Remove extra fdt_fixup_ethernet() call")
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: York Sun <york.sun@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>