Documentation Error Performance Counters

[Steinegger]

In the doc/user_manual.doc on page 28 there's and error regarding the CSR addresses for the PCMR and PCER performance registers. As noted in the table on page 21 the the rtl/include/riscv_defines.sv file the addresses should be 0x7E instead of 0x7A.

https://github.com/pulp-platform/riscv/blob/3c3400b0e6681d3cf20d0463e3d9d1fa021296fc/rtl/include/riscv_defines.sv#L440-L446

[davideschiavone]

61593531b7cbb22549e3d18d704b509c450ec412