ARM: msm: platsmp: determine number of CPU cores at boot time

Previously we just assumed there were CONFIG_NR_CPUS cpus present in
the system. Instead, figure out the number of cpus from the MIDR
register.

Signed-off-by: Jeff Ohlstein <johlstei@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>
[arnd: clarified patch title]
Signed-off-by: Arnd Bergmann <arnd@arndb.de>