aarch64: vp8: Move the vp8dsp makefile entries to the right places

Even if NEON would be disabled, the init functions should be built
as they are called as long as ARCH_AARCH64 is set.

These functions are part of a generic DSP subsytem, not tied directly
to one decoder. (They should be built if the vp7 decoder is enabled,
even if the vp8 decoder is disabled.)

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit b4b27dce95a6d40bfcd78043d3abec7d80dae143)
