For some very annoying reason, Altera has never supported using textio for initialising ROMs. It supports $readmemh in verilog, and Xilinx has supported textio for rom initialisation for years. Your only options in VHDL are:
1. Define the ROM as a constant (maybe having the VHDL generated from some other source)
2. Define a function to initialise the ROM. If it is a mathematical function, it should be fairly straight forward to write in VHDL.
3. Use a .mif or .hex file. If you need to simulate the design, you will need to use a megawizard/IP catalog generated ROM as using the infered rom with attributes will not load the contents.