gpio: sprd: Change to use SoC compatible string

Change to use SoC compatible string instead of wildcard string.

The Spreadtrum SC9860 platform device trees and drivers' development
are still in progress, and now we will always recompile and ship
device trees at the same time as we will compile and ship the kernel,
so we do not need care about the backwards compatibility.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>