dt-bindings: vendor-prefixes: Clarify Macronix prefix

When looking at compatible prefixes, Macronix is sometimes referred as
"mxicy":
- mxicy,mx25r1635f
- mxicy,mx25u6435f
- mxicy,mx25v8035f
- mxicy,mx25f0a-spi
and sometimes as "mxic":
- mxic,multi-itfc-v009-nand-controller
- mxic,enable-randomizer-otp

The oldest prefix that is also the one preferred by Macronix engineers
is "mxicy", so document the other one and mark it deprecated.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-7-miquel.raynal@bootlin.com