ARM: ux500: Remove DB8500 PRCMU platform registration when DT is enabled

Now the DB8500 has Device Tree support it will be probed when the DT
is parsed, rendering the requirement for platform registration void.
This patch removes DB8500 PRCMU platform registration.

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>