When creating G0 layer, some regi2c_*.h headers were moved out from
esp_hw_support (G1) to soc (G0). In order to be consistent with that change,
move all the remaining regi2c_*.h headers to soc too.
This commit updates the visibility of various header files and cleans up
some unnecessary inclusions. Also, this commit removes certain header
include paths which were maintained for backward compatibility.