ARM: dts: msm: add support for DDR bw vote on sdxpoorwills target

To scale ddr based on the effective frequency of core enable
cpubw node, cpubw node will scale ddr based on the cpu core
frequency.

Change-Id: Iad5e98d60caa5516d98820b9d4563c7d2200ae94
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>