MIPS: generic: Update node names to avoid unit addresses

With the latest mkimage from U-Boot 2021.04, the generic defconfigs no
longer build, failing with:

/usr/bin/mkimage: verify_header failed for FIT Image support with exit code 1

This is expected after the linked U-Boot commits because '@' is
forbidden in the node names due to the way that libfdt treats nodes with
the same prefix but different unit addresses.

Switch the '@' in the node name to '-'. Drop the unit addresses from the
hash and kernel child nodes because there is only one node so they do
not need to have a number to differentiate them.

Cc: stable@vger.kernel.org
Link: https://source.denx.de/u-boot/u-boot/-/commit/79af75f7776fc20b0d7eb6afe1e27c00fdb4b9b4
Link: https://source.denx.de/u-boot/u-boot/-/commit/3f04db891a353f4b127ed57279279f851c6b4917
Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

MIPS: mscc: build FIT image for Luton

Luton now has already an u-boot port so let's build FIT images.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>