mfd: bd957x: Fix Kconfig dependency on REGMAP_IRQ

The BD957x driver uses REGMAP_IRQ but does not 'select' to depend on
it. This can cause build failures.  Select REGMAP_IRQ for BD957X.

Fixes: 0e969260

 ("mfd: bd9576: Add IRQ support")
Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/Y3SdCWkRr1L64SWK@dc75zzyyyyyyyyyyyyydt-3.rev.dnainternet.fi