mfd: db8500-prcmu: Rename register header

Drop the ambition to support dbx500, the other SoCs in this series
were never deleted and the support for them has been deleted.
DB8500 is what we support.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>