scripts/make_fit: Drop fdt image entry compatible string

According to the FIT image source file format document found in U-boot [1]
and the split-out FIT image specification [2], under "'/images' node" ->
"Conditionally mandatory property", the "compatible" property is described
as "compatible method for loading image", i.e., not the compatible string
embedded in the FDT or used for matching.

Drop the compatible string from the fdt image entry node.

While at it also fix up a typo in the document section of output_dtb.

[1] U-boot source "doc/usage/fit/source_file_format.rst", or on the website:
    https://docs.u-boot.org/en/latest/usage/fit/source_file_format.html
[2] https://github.com/open-source-firmware/flat-image-tree/blob/main/source/chapter2-source-file-format.rst



Fixes: 7a23b027 ("arm64: boot: Support Flat Image Tree")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>