dt-bindings: pinctrl: bcm4708-pinmux: rework binding to use syscon

As pointed by Rob, CRU is a kind of block that can't be guaranteed to
have everything exposed as subnodes. It's a set of various registers
that aren't tied to any single device. It could be described much more
accurately as MFD (Multi-Function Device).

Some hardware blocks may indeed want to access a register or two of the
CRU which requires describing it as the "syscon".

While at it replace exmple node name with the standard "pinctrl" (also
pointed out by Rob).

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