ARM: dts: imx6q-tbs2910: fix fec reset polarity

According to Documentation/devicetree/bindings/net/fsl-fec.txt the polarity
of "phy-reset-gpios" is assumed to be active-low unless a separate property
"phy-reset-active-high" is available. So replace the inconsistent polarity
description to make the correct active-low reset behavior more obvious.

Signed-off-by: Soeren Moch <smoch@web.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>