ARM: dts: imx6ull-phytec-tauri: Remove invalid property

The 'rs485-rts-active-high' property is not a valid property and is not
documented anywhere.

Remove it to fix the following DT schema warning:

serial@21f0000: Unevaluated properties are not allowed ('rs485-rts-active-high' was unexpected)

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>