dt-bindings: merge ina209 binding into ina2xx binding

The ina209 binding only differs from other ina2xx bindings in the
compatible string, so add it to the common binding and remove the ina209
binding file.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>