ARM: ux500: Remove PMU platform registration when booting with DT

PMU registration is successfully completed by Device Tree now, so
there is no longer a requirement to register it from platform code.
This patch removes platform registration during a DT boot.

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