mfd: bd9571mwv: Use devm_regmap_add_irq_chip()

Use devm_regmap_add_irq_chip() to simplify the code.

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