powerpc/setup_64: Make some routines static

The following routines are only called by local services and do not
need to be external symbols.

It fixes these W=1 errors :

../arch/powerpc/kernel/setup_64.c:261:13: error: no previous prototype for ‘record_spr_defaults’ [-Werror=missing-prototypes]
  261 | void __init record_spr_defaults(void)
      |             ^~~~~~~~~~~~~~~~~~~
../arch/powerpc/kernel/setup_64.c:1011:6: error: no previous prototype for ‘entry_flush_enable’ [-Werror=missing-prototypes]
 1011 | void entry_flush_enable(bool enable)
      |      ^~~~~~~~~~~~~~~~~~
../arch/powerpc/kernel/setup_64.c:1023:6: error: no previous prototype for ‘uaccess_flush_enable’ [-Werror=missing-prototypes]
 1023 | void uaccess_flush_enable(bool enable)
      |      ^~~~~~~~~~~~~~~~~~~~

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-7-clg@kaod.org