kconfig: serial: esp32: Make driver depend on SOC_ESP32
kconfig: serial: esp32: Make driver depend on SOC_ESP32

The ESP32 UART driver can only be used on the ESP32 SoC AFAICT. But it
did not have a depend clause so it was polluting the options of other
platforms.

With this patch the ESP32 driver will no longer be exposed to users of
non-ESP32 devices.

Signed-off-by: Sebastian Bøe <edee888afd63e94916688495334700803eba20a0@nordicsemi.no>

Vudentz/zephyr,ldts/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,kraj/zephyr,zephyriot/zephyr,galak/zephyr,fbsder/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,mbolivar/zephyr,zephyriot/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,punitvara/zephyr,explora26/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,galak/zephyr,aceofall/zephyr-iotos,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,ldts/zephyr,galak/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,kraj/zephyr,aceofall/zephyr-iotos,rsalveti/zephyr,fbsder/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3220,kraj/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,finikorg/zephyr,runchip/zephyr-cc3220,rsalveti/zephyr,ldts/zephyr,fbsder/zephyr,finikorg/zephyr,punitvara/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,aceofall/zephyr-iotos,mbolivar/zephyr,explora26/zephyr,rsalveti/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyriot/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr