arch: xtensa: Add ESP32 SoC

Due to the configurable nature of the Xtensa platform, the generic name of
"LX6" cannot be used to describe an SoC as far as Zephyr goes.  So ESP32 is
defined both as a SoC and as a board.

This is based on work by Rajavardhan Gundi.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>