ARM: create .secure_stack section only for PSCI

Jon Master reports that QEMU refuses to load a U-Boot image built
with CONFIG_ARMV7_NONSEC, but without CONFIG_ARMV7_PSCI since
commit 5a3aae68 ("ARM: armv7: guard memory reserve for PSCI
with #ifdef CONFIG_ARMV7_PSCI").

It looks like only PSCI that needs the Secure stack, so move
the #ifdef to guard the whole of .secure_stack allocation in order
not to create the empty section.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reported-by: Jon Masters <jcm@redhat.com>
Link: http://patchwork.ozlabs.org/patch/664025/