soc: bcm: brcmstb: pm: Add support for newer rev B3.0 controllers

Update the Device Tree binding document and add a matching entry for the
MEMC DDR controller revision B3.0 which is found on chips like 7278A0
and newer.

Signed-off-by: Doug Berger <opendmb@gmail.com>
[florian: tweak commit message, make it apply to upstream kernel]
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>