ls1028a: only include drivers/net/fsl_enetc.h when driver is compiled

ls1028a: only include drivers/net/fsl_enetc.h when driver is compiled

As hinted by its path, it's not really meant to be included outside of
the driver itself. This header uses CONFIG_SYS_RX_ETH_BUFFER which we
are trying to move under CONFIG_NET dependency. This file here can be
compiled without network support so make sure this only gets included
when needed.

The function from that header (fdt_fixup_enetc_mac) is already guarded
by CONFIG_FSL_ENETC so simply guard the inclusion of the header the same
way.

This was tested by building ls1028aqds_tfa_defconfig with
CONFIG_MSCC_FELIX_SWITCH and CONFIG_FSL_ENETC disabled.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>