da850: add new config file for AM18xx

add new configuration file da850_am18xxevm.h for AM18xx boards
which are based on da850 SOC. AM18xx has WINBOND spi flash which
is indicated in the config file. And make appropriate changes in
board.cfg for building.

Signed-off-by: Nagabhushana Netagunte <nagabhushana.netagunte@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>