ARM: shmobile: r8a7779: Cleanup header file

Remove unused function prototypes from r8a7779.h and
make the functions static.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>