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

Documentation/admin-guide/devices.txt

@@ -2704,6 +2704,9 @@
 		    ...
 		 185 = /dev/ttyNX15		Hilscher netX serial port 15
 		 186 = /dev/ttyJ0		JTAG1 DCC protocol based serial port emulation
+
+		 If maximum number of uartlite serial ports is more than 4, then the driver
+		 uses dynamic allocation instead of static allocation for major number.
 		 187 = /dev/ttyUL0		Xilinx uartlite - port 0
 		    ...
 		 190 = /dev/ttyUL3		Xilinx uartlite - port 3