device-tree: nexus7: Add IMEM syscon and reboot reason support

This patch add the IMEM syscon memory region to the DT,
as well as addds support for the magic reboot reason
values that are written to the address for each mode.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Andy Gross <agross@codeaurora.org>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>