ARM: dts: imx6: phycore-som: Disable micro-SD write protection

The micro-SD card doesn't feature a write-protect pin. Set the
corresponding property in the devicetree to handle this behavior
correctly and suppress driver warnings.

Signed-off-by: Yunus Bas <y.bas@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>