Documentation for test-chip specific features

[jerryz123]

Impact: docs

Description We are slowly adding testchip-like features into chipyard designs. Some features that would be good to document are

• [ ] Loadmem support in our testchip_tsi implementation
• [ ] TestChip clock/reset handling (Currently the TileResetCtrl registers in testchipip)
• [ ] New testchip_tsi fesvr cmdline args (+init_write, +no_hart0_msip)
• [ ] BootROM and BootAddrReg (from testchipip)
• [ ] CustomBootPin (from testchipip)
• [ ] How to extend testchip_tsi with more custom behavior when +init_write is insufficient

[colinschmidt]

Also explaining what to do when +init_write isn't enough.