ARM: vfp: Use undef hook for handling VFP exceptions

Now that the VFP support code has been reimplemented as a C function
that takes a struct pt_regs pointer and an opcode, we can use the
existing undef_hook framework to deal with undef exceptions triggered by
VFP instructions instead of having special handling in assembler.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>