ARM: DTS: da850: fix missing #gpio-cells in gpio node

The gpio node is missing the mandatory property #gpio-cells, which is
causing runtime errors when using GPIOs e.g. with gpio-leds or gpio-keys:

"could not get #gpio-cells for /soc/gpio@1e26000"

This fixes the problem and adds the missing parameter.
The value is 2 according to the gpio-davinci.txt binding.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>