ARM: ux500: Move Snowball's thermal DT node into existing PRCMU one

When the thermal DT node was inserted a new PRCMU node was created;
however, one already exists in the Snowball DTS file. Here we
amalgamate the two into a single consolidated node.

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