dt-bindings: memory: tegra20: emc: Document new LPDDR2 sub-node

dt-bindings: memory: tegra20: emc: Document new LPDDR2 sub-node

Some Tegra20 boards don't have RAM code stored in NVMEM, which is used for
the memory chip identification and the identity information should be read
out from LPDDR2 chip in this case. Document new sub-node containing generic
LPDDR2 properties that will be used for the memory chip identification if
RAM code isn't available. The identification is done by reading out memory
configuration values from generic LPDDR2 mode registers of SDRAM chip and
comparing them with the values of device-tree 'lpddr2' sub-node.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211006224659.21434-8-digetx@gmail.com
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>