ARM: dts: imx28: rename mmc2_sck_cfg

This patch renames mmc2_sck_cfg in order to prepare for an alternative
muxing setup.

Signed-off-by: Michael Heimpold <michael.heimpold@i2se.com>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>