The ESP32 Ethernet Kit has only a single GPIO pin exposed to SAUL via
`saul_gpio`, but that GPIO doubles as PHY clock input when `esp32_eth`
is used. Hence, the `saul_gpio` dep should be optional and only kick
in when `esp32_eth` is not used.
The commit moves the header files from `boards/common/esp32s3/include` that can be used for all types of ESP32x SoCs to a new common ESP32x board module.
