doc: fit: add dm-verity boot parameter documentation

Add documentation for CONFIG_FIT_VERITY which allows U-Boot to
construct dm-mod.create= and dm-mod.waitfor= kernel command-line
parameters from dm-verity metadata embedded in FIT filesystem
sub-images.

The new document covers the relationship between FIT loadable indices
and the /dev/fitN block devices that the Linux uImage.FIT block driver
creates, provides a complete .its example with a dm-verity-protected
SquashFS root filesystem, describes all required and optional dm-verity
subnode properties and explains how mkimage generates the verity
metadata automatically.

dm-verity is only supported for external-data FIT images (mkimage -E);
mkimage aborts with an error if the flag is omitted.

Signed-off-by: Daniel Golle <[email protected]>
Reviewed-by: Simon Glass <[email protected]>

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