ARM: dts: pandora-common: define wl1251 as child node of mmc3

Since v4.7 the dma initialization requires that there is a
device tree property for "rx" and "tx" channels which is
not provided by the pdata-quirks initialization.

By conversion of the mmc3 setup to device tree this will
finally allows to remove the OpenPandora wlan specific omap3
data-quirks.

Fixes: 81eef6ca ("mmc: omap_hsmmc: Use dma_request_chan() for requesting DMA channel")
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Cc: <stable@vger.kernel.org> # v4.7+
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>