ARM: dts: vf610: Nomenclature fixup for PTC12 pin used in RMII mode.

This commit includes a minor nomenclature fixup for boards based on the
Freescale VF610 SoC and which make use of the alternate "RMII1_RXD1"
functionality for pin PTC12.  This brings the macro name in-line with
both the datasheet and other similar macros.

Signed-off-by: Cory Tusar <cory.tusar@pid1solutions.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>