arm_neon: Add an optimized routine for deinterleaving chroma

This supports conversion from NV12/21/16/24 to I420/YV12/I422/I444.

This avoids hitting swscale for the NV12->I420 conversion, for hw
decoders that return NV12/21 in combination with the android vout
in YUV mode.

Signed-off-by: Martin Storsjö <martin@martin.st>