Allow secondary cores to wake up in Aarch64

At boot time secondary cores may stand by in Aarch32 mode. Therefore if the
primary core has already switched to Aarch64 mode, they must be able to
switch to Aarch64 as well. As a consequence we must take care of HCR_EL2.RW
when there is no hypervisor to do so.

Fixes ARM-software/tf-issues#243

Signed-off-by: Gerald Lejeune <gerald.lejeune@st.com>