dts: bindings: gpio: Add binding for 96Boards Low speed 1.8v header

All 96Boards complying to the IE spec exposes either 40pin or 30pin
standard low speed connectors for peripheral connectivity. These
connectors are well defined and available in the IE spec. So, lets
create a devicetree binding for the 40pin header for the 1.8v IE
96Boards. This binding will be utilized by the 96Boards
for exposing the GPIO pins as nexus node as per the devicetree spec.

This will allow the shields and applications to use board independent
GPIO mapping.

Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>