davinci: DA850/OMAP-L138 EVM: get rid of DA850_UI_EXP config option

Get rid of DA850_UI_EXP config option since it is not used anywhere
else in code.

Instead make the UI expander choice menu dependent on the EVM
selection itself.

Also add help text indicating that UI board is actually detected
automatically.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>