ARM: PSCI: Switch to per-CPU target PC storage in secure data section

Now that we have a secure data section and space to store per-CPU target
PC address, switch to it instead of storing the target PC on the stack.

Also save clobbered r4-r7 registers on the stack and restore them on
return in psci_cpu_on for Tegra, i.MX7, and LS102xA platforms.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>