powerpc/44x: Make ppc44x_idle_init() static

The 44x/fsp2_defconfig build fails with:

  arch/powerpc/platforms/44x/idle.c:30:12: error: no previous prototype for ‘ppc44x_idle_init’ [-Werror=missing-prototypes]
  30 | int __init ppc44x_idle_init(void)
     |            ^~~~~~~~~~~~~~~~

Fix it by making ppc44x_idle_init() static.

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231129131919.2528517-4-mpe@ellerman.id.au