legal: Add LICENSES directory with MSLA Snapshot to comply REUSE

For the record the file has been snapshoted this way:

    sum="1cfe8601a36fbff16686cf84d36dbd20c4c691faa9c4672231ba2eefa6f37e1b"
    t='158' ; h='842' # 2025-01-31
    mkdir -p LICENSES ;
    lynx -dump https://www.silabs.com/about-us/legal/master-software-license-agreement  \
    | tail +161 | head -n 842 \
    | tee LICENSES/LicenseRef-MSL \
    | sha256sum | grep $sum

If project want to comply to REUSE Specification 3.0
then it should been tracked in sources too, eg:

    # SPDX-FileCopyrightText: Silicon Laboratories Inc. <https://www.silabs.com/>
    # SPDX-License-Identifier: LicenseRef-MSLA

Origin: SiliconLabs/wisun-br-linux#16
Fowarded: https://github.com/SiliconLabsInternal/silabs-internal-policies-and-guidelines/pull/3
Signed-off-by: Philippe Coval <philippe.coval@silabs.com>

Author: rzr