dt-bindings: hsi: add omap4 hsi controller bindings

Update omap-ssi binding document to also cover the HSI
compliant module from OMAP4.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Acked-by: Rob Herring <robh@kernel.org>
[dropped the omap.dtsi update and updated patch description accordingly]
Signed-off-by: Sebastian Reichel <sre@kernel.org>