dts: xea: Provide missing FEC required properties (mac0 and reg_fec_3v3)

dts: xea: Provide missing FEC required properties (mac0 and reg_fec_3v3)

After the commit (SHA1: 7d08ddd09b75e7a3c103cc0d0d3ed700287f268e) some
u-boot specific XEA FEC related properties have been replaced by ones
from the Linux kernel.

To be more specific - XEA board (and imx287 in general) has built L2
switch connected to FEC, which needs some special treatment.

In u-boot it is handled with 'mac0' node, whereas Linux uses dedicated
switch DTS node.

Signed-off-by: Lukasz Majewski <lukma@denx.de>