Documentation: msm: Update qcom,board-id property description

Update the device tree bindings documentation for the
qcom,board-id property to describe an updated version of
the property format, which includes fields for hardware
platform version.

Change-Id: I92193ad165d5c22f7b558f44d823684178fc0443
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>