ARM: PSCI: Add missing CONFIG_ARMV7_PSCI_NR_CPUS for PSCI enabled platforms

The original PSCI implementation assumed CONFIG_ARMV7_PSCI_NR_CPUS=4.
Add this to platforms that have not defined it, using CONFIG_MAX_CPUS if
it is defined, or the actual number of cores for the given platform.

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