pstore-ram: add Device Tree bindings

ramoops is one of the remaining places where ARM vendors still rely on
board-specific shims.  Device Tree lets us replace those shims with
generic code.

These bindings mirror the ramoops module parameters, with two small
differences:

(1) dump_oops becomes an optional "no-dump-oops" property, since ramoops
    sets dump_oops=1 by default.

(2) mem_type=1 becomes the more self-explanatory "unbuffered" property.

Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>