ARM: dts: imx6-tx6: specify ethernet phy reset post-delay

Under certain circumstances the ethernet PHY cannot be detected on
Ka-Ro electronics TX6 modules. Using a phy-reset-post-delay of at least
2ms alleviates this problem. Define it to 10ms to be on the safe side.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>