target/arm: Move vfp_reg_ptr() to translate-neon.c.inc

The function vfp_reg_ptr() is used only in translate-neon.c.inc;
move it there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210430132740.10391-10-peter.maydell@linaro.org