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>