memory: Extend of_memory with LPDDR3 support

Add AC timings information needed to support LPDDR3 and memory
controllers along with helpers to obtain it.  These will be necessary
for upcoming Exynos5422 Dynamic Memory Controller driver.

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