arm64/fpsimd: Make kernel_neon_ API _GPL

Currently for reasons lost in the mists of time the kernel_neon_ APIs are
EXPORT_SYMBOL() but the general policy for floating point usage is that it
should be GPL only given the non-standard runtime environment that holds
while it is in use and PCS impacts when code is compiled for FP usage.

Given the limited existing deployment of non-GPL modules for arm64 and the
fact that other architectures like x86 already make their equivalent
functions GPL only this is not expected to be disruptive to existing users.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Mark Brown <broonie@kernel.org>