ARM: shmobile: Remove unused declaration of r8a7778_add_standard_devices_dt()

The actual implementation was removed by commit 40216263

 ("ARM:
shmobile: Remove unused r8a7778 auxdata and callback") in v3.12.

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