Hi RD,
0x54 is the offset for the pad configuration register for pin GPMC_A5. You will find the complete list of pad configuration registers with offsets in the AM335X TRM Rev. K, Table 9-7 (starting at page 1364). Please note that these registers start from 0x800, so the full offset for pin GPMC_A5 is actually 0x854. Pins are named after their function in muxmode 0.