Documentation: devices.txt: Update ttyUL major number allocation details

Describe when uartlite driver uses static/dynamic allocation for major
number based on maximum number of uartlite serial ports.

Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@amd.com>
Link: https://lore.kernel.org/r/20231116134003.3762725-2-manikanta.guntupalli@amd.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>