dt-bindings: mfd: brcm,cru: Add clkset syscon

CRU has a shared register that is used e.g. to control USB 2.0 PHY block
access. It's a single 32 b register. Document it as syscon so it can be
used with a regmap.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>