net: fm: disables unused FM1-DTSEC1 MAC node in DTS

We don't disable unused FM1-DTSEC1 MAC node in FMAN v2 since it is
used by MDIO. For FMAN v3, MDIO uses dedicated controller, so we
can disable unused FM1-DTSEC1 MAC node to avoid being probed in
Linux.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
[York Sun: revised commit message]
Reviewed-by: York Sun <yorksun@freescale.com>