Rocket chip only has 245MB memory on FPGA

**What is the current behavior?**
I tried to run rocket chip on both ZC706 and Zedboard, but it seems that the rocket chip only has 245MB memory on both fpga boards (I ran riscv-linux on rocket chip, and the riscv-linux says there is 245MB memory).

**What is the expected behavior?**
How to change rocket chip memory according to the maximum available memory on FPGA board? Thanks!

The issue names no files, tests, or entry points. Start by reproducing riscv-linux on the ZC706 and Zedboard, then trace how Rocket Chip configures FPGA memory; done means the reported memory matches the board's intended available capacity.

Written by the indexing model from the issue text.