Documentation: dt: device tree bindings for LPDDR2 memories

device tree bindings for LPDDR2 SDRAM memories compliant
to JESD209-2 standard.

The 'lpddr2' binding in-turn uses another binding 'lpddr2-timings'
for specifying the AC timing parameters of the memory device at
different speed-bins.

Reviewed-by: Benoit Cousson <b-cousson@ti.com>
Reviewed-by: Grant Likely <grant.likely@secretlab.ca>
Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Aneesh V <aneesh@ti.com>
[santosh.shilimkar@ti.com: Rebased against 3.6-rc]
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Benoit Cousson <b-cousson@ti.com>