of: of_spmi: Add support for spmi-dev-container binding

The spmi-dev-container binding is intended for SPMI
configurations that have multiple device nodes associated with
only one spmi_device. By default, if this flag is not specified,
each device node will create a new spmi_device.

Sometimes having multiple spmi_devices for SPMI device nodes is
superfluous. One example of this is gpios. In some architectures,
a single gpio is treated as a unique device. But from a gpio_chip
perspective, the chip is comprised of many gpios. Beyond wasting
memory allocating a unique spmi_device per gpio, the implication
of not coalescing spmi_devices is that the clients probe() routine
would be called N number of times. But this sort of behavior makes
it difficult to realize when a gpio_chip starts and stops. If we
assume that one gpio_chip represents one call to probe(), then
this problem is solved, since all gpios in that chip will be
passed as resources.

In order to support multiple device nodes per spmi_device, we
also need to extend the data structures for spmi_resources.

This change also makes an effort to cleanup some of the error
handling for illegal combinations of device bindings, as well as
adding some additional documentation.

Change-Id: If3ce2aaaa07bdf79e0d9fdedf16419e74a00fbec
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>