Declare a ramoops memory segment to aid debugging for those without UART
access. Verified to carry console log when holding volume down for 15
seconds.

No memory region for ramoops-like support was found downstream, so the
arbitrarily picked region is the last MB of System RAM.

Cc: John Stultz <john.stultz@linaro.org>
Cc: Mart Raudsepp <leio@gentoo.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>