soc: ti: k3: Enable BUILD_NO_GAP_FILL for all M4

By default, a post build step tries to fill gaps in the output
hex file. Since the AM62x, for example, has different,
non-contiguous memory sections, it tries to fill a gap over
multiple GBs. Disable this feature since the K3 architecture has no
dedicated flash for the firmware and stores it in a Linux rootfs.

Signed-off-by: Daniel Schultz <d.schultz@phytec.de>