ARM: dts: msm: update DDR BW voting on SDM660 target

SDM660 target has a dual DDR channel of width 16 bits.
Update DDR BW voting considering per channel voting.

Change-Id: I760e0149c75c7968e307d3ca71a5d7b7985c3b32
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>