powerpc/512x: Make pdm360ng_init() static

The mpc512x_defconfig config fails with:

  arch/powerpc/platforms/512x/pdm360ng.c:104:13: error: no previous prototype for ‘pdm360ng_init’ [-Werror=missing-prototypes]
  104 | void __init pdm360ng_init(void)
      |             ^~~~~~~~~~~~~

Fix it by making pdm360ng_init() static.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231129131919.2528517-2-mpe@ellerman.id.au