powerpc: Use str_plural() in cpu_init_thread_core_maps()

Fixes the following Coccinelle/coccicheck warning reported by
string_choices.cocci:

	opportunity for str_plural(tpc)

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240331222249.107467-2-thorsten.blum@toblux.com