dt-bindings: memory: lpddr3: adjust IO width to spec

According to JEDEC Standard No. 209-3 (table 3.4.1 "Mode Register
Assignment and Definition in LPDDR3 SDRAM"), the LPDDR3 supports only
16- and 32-bit IO width.  Drop the unsupported others.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220206135807.211767-5-krzysztof.kozlowski@canonical.com