cpufreq, powernv: Fix build failure on UP

Paul Gortmaker reported the following build failure of the powernv cpufreq
driver on UP configs:

drivers/cpufreq/powernv-cpufreq.c:241:2: error: implicit declaration of
function 'cpu_sibling_mask' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
make[3]: *** [drivers/cpufreq/powernv-cpufreq.o] Error 1
make[2]: *** [drivers/cpufreq] Error 2
make[1]: *** [drivers] Error 2
make: *** [sub-make] Error 2

The trouble here is that cpu_sibling_mask is defined only in <asm/smp.h>,
and <linux/smp.h> includes <asm/smp.h> only in SMP builds.

So fix this build failure by explicitly including <asm/smp.h> in the driver,
so that we get the definition of cpu_sibling_mask even in UP configurations.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>