ARM: armv7: move ARMV7_PSCI_NR_CPUS to Kconfig

Move this option to Kconfig and set its default value to 4; this
increases the number of supported CPUs for some boards.

It consumes 1KB memory per CPU for PSCI stack, but it should not
be a big deal, given the amount of memory used for the modern OSes.

Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>