board: sl28: fix network on variant 3

board: sl28: fix network on variant 3

Network is broken on variant 3 boards since commit 61ff13283c3b ("board:
sl28: move to OF_UPSTREAM") because it was removing the variant 3
handling. That is because at that time the var3 device tree was not
upstream. FWIW variant 3 is actually the same as the base variant, but
I've missed that the -u-boot.dtsi is not inlcuded in this case which
will set the ethernet alias.  Now that the var3 device tree is upstream,
just re-add it to the SPL handling again.

Fixes: 61ff13283c3b ("board: sl28: move to OF_UPSTREAM")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Reviewed-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>