ARM: dts: da850,da850-evm: Add an aemif node and use it for the NAND

Currently the davinci da8xx boards use the mach-davinci aemif code.
Instantiating an aemif node into the DT allows to use the ti-aemif
memory driver and is another step to better DT support.
This change adds an aemif node in the dtsi while retiring the nand_cs3
node. The NAND is now instantiated in the dts as a subnode of the aemif
one along with its pins.

Signed-off-by: Karl Beldan <kbeldan@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>