core: user_ta: use TEE_MMU_UMAP_STACK_IDX
core: user_ta: use TEE_MMU_UMAP_STACK_IDX

Use TEE_MMU_UMAP_STACK_IDX to replace the number 0.

Signed-off-by: Peng Fan <d3c162a96b7fb54e5d94bc77bc2c7cbbce10806f@nxp.com>
Acked-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os