dt-bindings: net: tja11xx: add nxp,refclk_in property

TJA110x REF_CLK can be configured as interface reference clock
intput or output when the RMII mode enabled. This patch add the
property to make the REF_CLK can be configurable.

Signed-off-by: Wei Fang <wei.fang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>