fdt: Add device tree memory bindings

Support a default memory bank, specified in reg, as well as
board-specific memory banks in subtree board-id nodes.

This allows memory information to be provided in the device tree,
rather than hard-coded in, which will make it simpler to handle
similar devices with different memory banks, as the board-id values
or masks can be used to match devices.

Signed-off-by: Michael Pratt <mpratt@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Vadim Bendebury <vbendeb@chromium.org>