i.MX6: Documentation: update from fsl,card-wired to non-removable

With the commit 7f217794 (mmc: dt: Consolidate DT bindings),
the device tree properties used by various device drivers
for SD/MMC host controllers were standardized.  One of the changes
was that the property "fsl,card-wired", previously used by the
Freescale driver, was replaced with "non-removable". Fix the example
documentation regarding this.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>