dt-bindings: pinctrl: bcm4708-pinmux: improve example binding

Broadcom SoC pins are controlled using CRU ("Clock and Reset Unit" or
"Central Resource Unit") registers. There are more CRU registers and
functions so CRU should be represented as a separated block in DT.

Moreover CRU is a sub-block of DMU ("Device Management Unit") so that
one should also get its own node.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>