trats2: defconfig: disable memset at malloc init

Reduce the boot time of Trats2 by disabling the memset
at malloc init.

This was tested on Trats2.
A quick test with trace. Boot time from start to main_loop() entry:
- ~464ms - before this change (arch memset enabled for .bss clear)
- ~341ms - after this change

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>