dt-bindings: ddr: Add bindings for Samsung LPDDR3 memories

Add compatible for Samsung k3qf2f20db LPDDR3 memory bindings, based on
at25.txt compatible section.  Introduce minor fixes in the old
documentation.

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