Fix TEE_BigIntMulMod

Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org>
Signed-off-by: Pascal Brand <pascal.brand@st.com>
Remove unused function tee_pobj_init

Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Remove unused chip_services.c

Removes chip_services.c and declaration of enable_secure_wd() in
chip_services.h

Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Jens Wiklander <jens.wiklander@linaro.org>