xtensa: xtfpga: only select ethoc when ethernet is available

Otherwise we get the following build warning:
    (XTENSA_PLATFORM_XTFPGA) selects ETHOC which has unmet direct
    dependencies (NETDEVICES && ETHERNET && HAS_IOMEM && HAS_DMA)
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/Kconfig

@@ -319,8 +319,8 @@ config XTENSA_PLATFORM_S6105
 
 config XTENSA_PLATFORM_XTFPGA
 	bool "XTFPGA"
+	select ETHOC if ETHERNET
 	select SERIAL_CONSOLE
-	select ETHOC
 	select XTENSA_CALIBRATE_CCOUNT
 	help
 	  XTFPGA is the name of Tensilica board family (LX60, LX110, LX200, ML605).