ARM: ux500: Reduce PRCMU reg-names to shorter form for u8540 DT

After some discussion and deliberation we have decided to only use the
short form of the PRCMU register names i.e. not mention the peripheral
in which the registers reside.

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