mediatek: mt7623: add scatter file for unbricking with SP Flash Tool

mediatek: mt7623: add scatter file for unbricking with SP Flash Tool

Many MediaTek SoCs can be unbricked by using the SP Flash Tool from
http://spflashtool.com/ along with a "scatter list" file, which is
just a text file listing which image gets loaded where.

We use a trivial partition layout for the tool, with the whole eMMC
image as a single "partition", which means users just need to unzip
the sysupgrade image. Doing the real partition layout would be overly
complex and would require the individual partitions to be shipped
as artifacts — or users to extract them out of the sysupgrade image
just for the tool to put them adjacent to each other on the eMMC
anyway.

The tool does require a copy of the preloader in order to operate,
even when it isn't flashing the preloader to the eMMC boot region.
So drop that into the bin directory as an artifact too.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>