openrisc: Use devicetree to determine present cpus

Use the device tree to determine the present cpus instead of assuming
all CONFIG_NRCPUS are actually present in the system.

Signed-off-by: Jan Henrik Weinstock <jan.weinstock@rwth-aachen.de>
[shorne: Squashed 2 email commits and added summary from email]
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>