of: spmi: Verify that nodes are available before creating devices

Device Tree allows for devices to be specified as disabled.
This is useful to override SoC specific configuration on boards
that may not have this device.

Change-Id: I77490f6d2f2634c526cf01a2b0da029ff0d51ef0
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>