These devices are identified in user space by a filename: if you want to read from a UART, you open the device node, for example, the first serial port on the ARM Versatile Express would be /dev/ttyAMA0. The driver is identified differently in the kernel, using the major number which, in the example given, is 204. Since the UART driver can handle more than one UART, there is a second number, called the minor number, which identifies a specific interface, 64, in this case:
# ls -l /dev/ttyAMA* crw-rw---- 1 root root 204, 64 Jan 1 1970 /dev/ttyAMA0 crw-rw---- 1 root root 204, 65 Jan 1 1970 /dev/ttyAMA1 crw-rw---- 1 root root 204, 66 Jan 1 1970 /dev/ttyAMA2 crw-rw---- 1 root root 204, 67 Jan 1 1970 /dev/ttyAMA3
The list of standard major and minor numbers can be found in the kernel documentation, in Documentation/devices.txt. The list does not get updated very often and does not include the ttyAMA device described...