powerpc/embedded6xx: Make functions flipper_pic_init() & ug_udbg_putc() static

Change signature of two functions, adding static keyword to prevent the
following two warnings (treated as errors on W=1):

  arch/powerpc/platforms/embedded6xx/flipper-pic.c:135:28: error: no previous prototype for ‘flipper_pic_init’
  arch/powerpc/platforms/embedded6xx/usbgecko_udbg.c:172:6: error: no previous prototype for ‘ug_udbg_putc’

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