fdt: Add function for decoding multiple gpios globally available

Samsung's SDHCI bindings require multiple gpios to be parsed and
configured at a time. Export the already available fdtdec_decode_gpios
for this purpose.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Commit-Ready: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>