drm/amd/pm: Replace one-element array with flexible-array in struct phm_clock_array

drm/amd/pm: Replace one-element array with flexible-array in struct phm_clock_array

There is a regular need in the kernel to provide a way to declare having
a dynamically sized set of trailing elements in a structure. Kernel code
should always use “flexible array members”[1] for these cases. The older
style of one-element or zero-length arrays should no longer be used[2].

Refactor the code according to the use of a flexible-array member in
struct phm_clock_array, instead of a one-element array, and use the
struct_size() helper to calculate the size for the allocation.

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arrays

Build-tested-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/5f7c433f.ZyMD+YUIVAwiHGVe%25lkp@intel.com/
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>