configs: ls1028a: ensure Ethernet is enabled

CONFIG_FSL_ENETC is not explicitly enabled in the NXP LS1028A config
files, instead it is selected by CONFIG_MSCC_FELIX_SWITCH, a state of
matters which is fragile.

CONFIG_MSCC_FELIX_SWITCH depends on CONFIG_DM_DSA, which depends on
CONFIG_PHY_FIXED.

Not all LS1028A boards did enable CONFIG_PHY_FIXED, which resulted in
all of Ethernet being compiled out.

This patch makes sure that CONFIG_PHY_FIXED is enabled for all LS1028A
boards, and CONFIG_FSL_ENETC as well - don't rely on that fragile
selection done by the Felix switch config.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>