arm64: fpsimd: Abstract out binding of task's fpsimd context to the cpu.

There is currently some duplicate logic to associate current's
FPSIMD context with the cpu when loading FPSIMD state into the cpu
regs.

Subsequent patches will update that logic, so in order to ensure it
only needs to be done in one place, this patch factors the relevant
code out into a new function fpsimd_bind_to_cpu().

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>