MAINTAINERS: Adjust ARM/NOMADIK/Ux500 ARCHITECTURES to file renaming

Commit 8f00b3c4 ("mfd: db8500-prcmu: Rename register header") renames
dbx500-prcmu-regs.h to db8500-prcmu-regs.h in drivers/mfd/ but misses
to adjust the ARM/NOMADIK/Ux500 ARCHITECTURES section in MAINTAINERS.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/mfd/dbx500*

Remove the obsolete file entry after this file renaming.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>