powerpc/embedded6xx: Make some functions static

In commit cb9e4d10

 ("[POWERPC] Add support for 750CL Holly board")
new functions were added. Since most of these functions can be made
static, make it so.

Both holly_power_off and holly_halt functions were not changed since
they are unused, making them static would have triggered the following
warning (treated as error):

  arch/powerpc/platforms/embedded6xx/holly.c:244:13: error: 'holly_halt' defined but not used

Silence the following warnings triggered using W=1:

  arch/powerpc/platforms/embedded6xx/holly.c:47:5: error: no previous prototype for 'holly_exclude_device'
  arch/powerpc/platforms/embedded6xx/holly.c:190:6: error: no previous prototype for 'holly_show_cpuinfo'
  arch/powerpc/platforms/embedded6xx/holly.c:196:17: error: no previous prototype for 'holly_restart'

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>