psci: export psci_is_last_on_cpu function

psci_is_last_on_cpu() function becomes public.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Change-Id: I393219d0ea01207c08289e91d96f0a33888de10f
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/176501


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: CIBUILD <smet-aci-builds@lists.codex.cro.st.com>
Reviewed-by: Lionel DEBIEVE <lionel.debieve@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>