hwspinlock: document the hwspinlock 'raw' API

Document the hwspin_lock_timeout_raw(), hwspin_trylock_raw() and
hwspin_unlock_raw() API.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>