Documentation: dt: OMAP: explicitly state SoC compatible strings

SoC family definitions at the moment are reactive to board needs.
This results in potentially wrong matches taking place for board types.

Eventually, we will have descriptors match only with SoC types and
should not contain anything specific to board handling and pave the
way to getting rid of soc_is_XYZ checks and allow this determination
done with matches at of level.

Existing implicit definitions(as part of board descriptions) are
explicitly documented.

NOTE: Even though we prefer to have dt compatibility to explicit,
"ti,dra7xx", "ti,am33xx", "ti,omap36xx" preexist in current dts and
are maintained for backward compatibility. It is also expected that
any future SoC addition will keep this documentation updated.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>