ARM: shmobile: apmu: Remove platsmp-apmu.h

After the removal of the legacy SMP fallbacks, platsmp-apmu.h is no
longer needed outside platsmp-apmu.c.

Hence remove platsmp-apmu.h, and make the functions exported previously
static.  As the header file also provided forward declarations, the code
in platsmp-apmu.c must be reshuffled.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>