Fix DBGR ISP and update docs

[crawfxrd]

In my experience, external flashing using DBGR/EPP has been unreliable since the TGL boards.
Following the docs, ecflash either times out or reads an invalid chip ID a majority of the time.

[crawfxrd]

Need to check:

• DBGR clock gating is enabled (CGCTRL3[0]=1): Should be ungated when trying to do ISP?
• DBGR has special requirements for entry:
  • DBGR/EPP: KSI straps must be configured a certain way
    • Ref: IT5570 spec, Table 5-21. Pin Descriptions of Hardware Strap
  • DBGR/SMB: requires a special non-standard I2C sequence, per Google's docs
    • Ref: util/iteflash.md#how-ite-ec-reflashing-works