of: of_spmi: Change name of spmi-dev-container

This binding is currently used to indicate all devices existing
in the same slave. Let's change the name to be more meaningful.
The real motivation here is that we want to introduce a new
binding to specify all qpnp devices existing in the same
spmi_device. So spmi-dev-container is a more meaningful name for
that usecase, and spmi-slave-container better describes the
former.

Change-Id: I48f834b9cff9ea90d05f5e958ca21bef0ab56a86
Signed-off-by: Michael Bohan <mbohan@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>