samples: openamp: espressif: move configs from SoC to board level

Previously, the openamp sample enabled ESP32/ESP32S3 SoCs directly,
which caused all SoCs of these families to be included in the testcase.
However, only a subset of boards is actually supported and mapped in
Kconfig.sysbuild.

To avoid applying defaults to unsupported boards, move the configuration
out of the SoC directory and into board-specific files. This ensures that
only the intended boards are targeted by the openamp sample and avoids
misleading test coverage.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>