ARM: OMAP1: ams-delta: Move AMS_DELTA_LATCH2_NGPIO to the board file

That symbol is not used outside the board file, there is no need to
keep it in the board header.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>