spl: binman: Disable u_boot_any symbols for i.MX8M boards

spl: binman: Disable u_boot_any symbols for i.MX8M boards

The i.MX8M boards use partially specified binman images which have an
SPL entry without a U-Boot entry. This would normally cause an error due
to the 'u_boot_any' binman symbols declared by BINMAN_UBOOT_SYMBOLS
requiring a U-Boot-like entry in the same image as the SPL.

However, a problem in the ARMv8 __image_copy_start symbol definition
effectively disables binman from attempting to write any symbols at all,
so everything appears to work fine until runtime. A future patch fixes
the issue in the linker scripts, which lets binman fill in the symbols,
which would result in the build error described above.

Explicitly disable the 'u_boot_any' symbols for i.MX8M boards. They are
already effectively unusable, and they are incompatible with the boards'
current binman image descriptions.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>