mfd: bd9571mwv: Use the SPDX license identifier

Use the SPDX license identifier instead of a local description.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-for-MFD-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>