More dts changes for omaps for v5.6 merge window

Add basic support for first generation Amazon omap3-echo. This got
applied rather late as we discussed how to deal with SoC variants
with some accelerators unaccessible, and eventually ended up setting
up few more SoC specific dtsi files. Eventually we'll need to also
detect the disabled accelerators on driver init, but more patching
is needed for that.
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