dt-bindings: mfd: Add Broadcom CRU

CRU is a block used in e.g. Northstar devices. It can be seen in the
bcm5301x.dtsi and this binding documents its proper usage.

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