There're 2 versions of motherboards that could be used in ARC SDP.
The only important difference for U-Boot is different NAND IC in use:
 [1] v2 board (we used to support up until now) sports MT29F4G08ABADAWP
while
 [2] v3 board sports MT29F4G16ABADAWP

They are almost the same except data bus width 8-bit in [1] and 16-bit
in [2]. And for proper support of 16-bit data bus we have to pass
NAND_BUSWIDTH_16 option to NAND driver core - which we do now knowing
board type we're running on.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>