pstore: Make ramoops_init_przs generic for other prz arrays

Currently ramoops_init_przs() is hard wired only for panic dump zone
array. In preparation for the ftrace zone array (one zone per-cpu) and pmsg
zone array, make the function more generic to be able to handle this case.

Heavily based on similar work from Joel Fernandes.

Signed-off-by: Kees Cook <keescook@chromium.org>