Fix sections to map

As highlighted by z08053520@126.com, MMU mapping functions tee_mmu_is_mapped()
and tee_mmu_map_io() wrongly compute the number of sections to check / map.

This patch checks / maps only the relevant sections.

Signed-off-by: Pascal Brand <pascal.brand@st.com>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)