[doc] Update configurations in README.md

[andreaskurth]

[The configurations in doc/02_user/configuration.rst] differ from those in the ReadMe. While updating the ReadMe is probably out of scope of this PR, should we create an issue to align it?

Originally posted by @andreaskurth in #1980 (comment)

Yes, we should. I think we can just switch to maxperf-pmp-bmbalance, though will need to get some synthesis numbers. We should add the 'micro' configuration too though I don't think that can be a supported configuration as I don't think riscv-dv supports it (I seem to remember there are some toolchain issues with E as well). Still we could do some verilator based smoke test for the micro config, ensure it's not entirely dead ;)

Originally posted by @GregAC in #1980 (comment)