Use flexible array to simplify memory allocation.
It saves some memory, avoids an indirection when reading the 'clusters'
array and removes some LoC.

Detailed explanation:
====================
Knowing that:
  - each devm_ allocation over-allocates 40 bytes for internal needs
  - Some rounding is done by the memory allocator on 8, 16, 32, 64, 96,
    128, 192, 256, 512, 1024, 2048, 4096, 8192 boundaries

and that:
  - sizeof(struct tegra186_cpufreq_data) = 24
  - sizeof(struct tegra186_cpufreq_cluster) = 16

Memory allocations in tegra186_cpufreq_probe() are:
  data:           (24 + 40) = 64 		      => 64 bytes
  data->clusters: (2 * 16 + 40) = 72     => 96 bytes
So a total of 160 bytes are allocated.
56 for the real need, 80 for internal uses and 24 are wasted.

If 'struct tegra186_cpufreq_data' is reordered so that 'clusters' is a
flexible array:
  - it saves one pointer in the structure
  - only one allocation is needed

So, only 96 bytes are allocated:
  16 + 2 * 16 + 40 = 88  => 96 bytes
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>