dt-bindings: clock: add rk3399 DDR3 standard speed bins.

DDR3 SDRAM Standard (JESD79-3F) defines some standard speed bins for
DDR3 memories. The rk3399_dmc driver allows you to pass these values via
the device tree. For that purpose the devfreq/rk3399_dmc.txt binding
refers to a ddr.h file which does not exist. This patch adds the missing
defines in a include file called rk3399-ddr.h with the definition of
standard speed bins according to the ARM Trusted Firmware (ATF).

Fixes: c1ceb8f7 (Documentation: bindings: add dt documentation for rk3399 dmc)
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>