[POWERPC] 83xx: Fix wrong USB phy type in mpc837xmds dts

Due to chip constraint MPC837x USB DR module can only use
ULPI and serial PHY interfaces.  The patch fixes the wrong
type in dts.

Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>