dt-bindings: mmc: clarify disable-wp text

"disable-wp" has been removed from all Rockchip eMMC and SDIO dts nodes,
but people still continue to submit new patches with "disable-wp" added
to other nodes then for the SD card slot,
what it was designed for in the first place.
So clarify the "disable-wp" text by adding that this option should
not be used in combination with eMMC or SDIO.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>