cpufreq: powernv: fix stack bloat and hard limit on number of CPUs

The following build warning occurred on powerpc 64-bit builds:

drivers/cpufreq/powernv-cpufreq.c: In function 'init_chip_info':
drivers/cpufreq/powernv-cpufreq.c:1070:1: warning: the frame size of
1040 bytes is larger than 1024 bytes [-Wframe-larger-than=]

This is with a cross-compiler based on gcc 8.1.0, which I got from:
  https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/8.1.0/

The warning is due to putting 1024 bytes on the stack:

    unsigned int chip[256];

...and it's also undesirable to have a hard limit on the number of
CPUs here.

Fix both problems by dynamically allocating based on num_possible_cpus,
as recommended by Michael Ellerman.

Fixes: 053819e0

 ("cpufreq: powernv: Handle throttling due to Pmax capping at chip level")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: 4.10+ <stable@vger.kernel.org> # 4.10+
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>