dt-bindings: brcm: rationalize Broadcom documentation naming

This patchset attempts to standardize the naming of dt-bindings
documents based on the Broadcom vendor prefix of brcm.

Although there are no guidelines currently present for how to name
the dt-bindings document the "vendor,binding.txt" style is in use by
some of the other vendors.

Acked-by: Lee Jones <[e-mail]>
Acked-by: Florian Fainelli <[e-mail]>
Acked-by: Gregory Fong <[e-mail]>
Acked-by: Stephen Warren <[e-mail]>
Signed-off-by: Scott Branden <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>