arm64: dts: sprd: Add GPIO and GPIO keys device nodes

This patch adds device nodes to enable one GPIO controller located on
digital chip, 2 EIC (external interrupt controller) controllers loacted
on PMIC and digital chip for Spreadtrum SC9860 platform.

Moreover this patch adds 3 GPIO keys relied on EIC controller to support
power key and volume up/down keys.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>