This commit converts mccmon6's u-boot proper (in a single commit to avoid
build breaks) to use solely DM/DTS.
The DTS description of the mccmon6 has been ported from Linux kernel
(v4.20, SHA1: 8fe28cb58bcb235034b64cbbb7550a8a43fd88be)
Signed-off-by: Lukasz Majewski <lukma@denx.de>