xilinx: Only enable dist boot pxe when DHCP is enabled

Otherwise, we see this:
In file included from include/configs/zynq-common.h:183:0,
                 from include/config.h:5,
                 from include/common.h:21,
                 from env/common.c:11:
include/config_distro_bootcmd.h:319:2: error: expected ?}? before ?BOOT_TARGET_DEVICES_references_PXE_without_CONFIG_CMD_DHCP_or_PXE?
  BOOT_TARGET_DEVICES_references_PXE_without_CONFIG_CMD_DHCP_or_PXE
  ^
include/config_distro_bootcmd.h:319:2: note: in definition of macro ?BOOTENV_DEV_NAME_PXE?

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>