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 <sebastian.boe@nordicsemi.no>