dt-bindings: mfd: Add Broadcom's MISC block

Broadcom's MISC is an MFD hardware block used on some of their SoCs like
bcm63xx and bcm4908. At this point only PCIe reset is fully understood
and documented. More functions may be added later.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>