The available UART speeds depend on 4 factors:
 
1.  main crystal speed, which is 22.118MHz
2.  VCO_MULTIPLIER, currently set to 22
3.  SCLK_DIVIDER, currently set to 4
4.  UART0_DIVIDER
 
The core CPU speed equals crystal * VCO_MULTIPLIER = 486.6MHz.  To set a baud rate, this number is divided by 4, which gives you the memory bus speed (121.65MHz), and then divided by 16, which gives you the maximum UART speed (7.6MHz).  All possible UART speeds are then derived from this maximum.  If the UART0_DIVIDER is set to 1, your UART speed is 7.6Mbps.  Set to 2, it's 3.8Mbps.  Set to 3, it's 2.53Mbps, etc.
 
You could try other combinations of VCO_MULTIPLIER and SCLK_DIVIDER to achieve other UART speed settings, but you need to be careful, as I don't know whether you will hit any issues if the core CPU speed goes beyond 500MHz or the memory bus speed goes beyond the current 121MHz setting.