powerpc/52xx: Add missing functions prototypes

The function prototypes were declared within a `#ifdef CONFIG_PPC_LITE5200`
block which would prevent them from being visible when compiling
`mpc52xx_pm.c`. Move the prototypes outside of the `#ifdef` block to fix
the following warnings treated as errors with W=1:

  arch/powerpc/platforms/52xx/mpc52xx_pm.c:58:5: error: no previous prototype for ‘mpc52xx_pm_prepare’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/52xx/mpc52xx_pm.c:113:5: error: no previous prototype for ‘mpc52xx_pm_enter’ [-Werror=missing-prototypes]
  arch/powerpc/platforms/52xx/mpc52xx_pm.c:181:6: error: no previous prototype for ‘mpc52xx_pm_finish’ [-Werror=missing-prototypes]

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>