Remove invalid DEPFILE option in CMake helper

In CMake versions < 3.31 specifying invalid options to
add_custom_command (and others) was silently ignored. Specifying the
DEPFILE option to a custom command (per target) has never been valid so
remove the option.

Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>
Change-Id: I2b57d286dffff4484905e7c63007caf23522b58d