arm64: add support for kernel mode NEON

arm64: add support for kernel mode NEON

Add <asm/neon.h> containing kernel_neon_begin/kernel_neon_end function
declarations and corresponding definitions in fpsimd.c

These are needed to wrap uses of NEON in kernel mode. The names are
identical to the ones used in arm/ so code using intrinsics or
vectorized by GCC can be shared between arm and arm64.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
(cherry picked from commit 4cfb36136480c029a29dbf63a623506e6ed7282b)
Signed-off-by: Mark Brown <broonie@linaro.org>