ti: am335x: Fix build when networking is disabled

When compiling without CONFIG_CLOCK_SYNTHESIZER (which is implied by
CONFIG_DRIVER_TI_CPSW for am335x_evm), exclude the network setup for
AM335x-ICEv2 to avoid link time failures:

  board/ti/am335x/board.c:683: undefined reference to `setup_clock_synthesizer'

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>