dt-bindings: ddr: Add bindings for LPDDR3 memories

Specifies the AC timing parameters of the LPDDR3 memory device.

Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>