ARM: ux500: Allow PRCMU regulator to be probed during a DT enabled boot

This patch adds the correct compatible string for use during Device Tree
population. Without it the DB8500 PRCMU regulators would be processed
when DT is enabled.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>