ARM: dts: am335x-sancloud-bbe: fix phy mode

commit c842b8c4 upstream.

The phy mode should be rgmii-id.  For some reason, it used to work with
rgmii-txid but doesn't any more.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>