stm32mp15: Avoid writing symbols in SPL

These boards use SPL in a mkimage entry and apparently access the symbol
containing the image position of U-Boot, but put U-Boot in another
image. This means that binman is unable to fill in the symbol correctly
in the SPL binary.

This doesn't matter at present since mkimage doesn't support symbol
writing. But with the upcoming conversion to a section, it will. So add
a property to disable symbol writing.

Signed-off-by: Simon Glass <sjg@chromium.org>