Documentation: dw-mshc: deprecate num-slots

dwmmc host driver already deprecate it in the driver
but didn't modify the documentation to reflect the fact.
This patch deprecates it and clean up num-slots from the
examples of all variant host drivers.

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Fixes: d30a8f7b ("mmc: dw_mmc: deprecated the "num-slots" property")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>