powerpc/83xx: Fix build failure with FPU=n

Building eg. 83xx/mpc832x_rdb_defconfig with FPU=n, fails with:

  arch/powerpc/platforms/83xx/suspend.c: In function 'mpc83xx_suspend_enter':
  arch/powerpc/platforms/83xx/suspend.c:209:17: error: implicit declaration of function 'enable_kernel_fp'
    209 |                 enable_kernel_fp();

Fix it by providing an enable_kernel_fp() stub for FPU=n builds,
which allows using IS_ENABLED(CONFIG_PPC_FPU) around the call to
enable_kernel_fp().

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240306125853.3714578-2-mpe@ellerman.id.au