ARM: allocate extra space for PSCI stack in secure section during link phase

The PSCI implementation expects at most 2 pages worth of space reserved
at the end of the secure section for its stacks. If PSCI is relocated to
secure SRAM, then everything is fine. If no secure SRAM is available,
and PSCI remains in main memory, the reserved memory space doesn't cover
the space used by the stack.

If one accesses PSCI after Linux has fully booted, the memory that should
have been reserved for the PSCI stacks may have been used by the kernel
or userspace, and would be corrupted. Observed after effects include the
system hanging or telinit core dumping when trying to reboot. It seems
the init process gets hit the most on my test bed.

This fix allocates the space used by the PSCI stacks in the secure
section by skipping pages in the linker script, but only when there is
no secure SRAM, to avoid bloating the binary.

This fix is only a stop gap. It would be better to rework the stack
allocation mechanism, maybe with proper usage of CONFIG_ macros and an
explicit symbol.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>