mfd: mfd-core: Honour Device Tree's request to disable a child-device

Until now, MFD has assumed all child devices passed to it (via
mfd_cells) are to be registered. It does not take into account
requests from Device Tree and the like to disable child devices
on a per-platform basis.

Well now it does.

Link: https://www.spinics.net/lists/arm-kernel/msg366309.html
Link: https://lkml.org/lkml/2019/8/22/1350Reported-by: Barry Song <Baohua.Song@csr.com>
Reported-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Stephan Gerhold <stephan@gerhold.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/mfd-core.c

@@ -152,6 +152,11 @@ static int mfd_add_device(struct device *parent, int id,
 	if (parent->of_node && cell->of_compatible) {
 		for_each_child_of_node(parent->of_node, np) {
 			if (of_device_is_compatible(np, cell->of_compatible)) {
+				if (!of_device_is_available(np)) {
+					/* Ignore disabled devices error free */
+					ret = 0;
+					goto fail_alias;
+				}
 				pdev->dev.of_node = np;
 				pdev->dev.fwnode = &np->fwnode;
 				break;