target/riscv: manage triggers available to OpenOCD for internal use

Before the change, if the user wrote to any `tdata*` register, OpenOCD
would sometimes start to disable all the triggers (by writing zeroes to
`tdata1`) and re-enable them again (by witing all trigger registers to the
values read before for each `tselect` value), e.g. on `step`
(see `disable/enable_triggers()`).

There are a couple of issues with such approach:
1. RISC-V Debug Specification does not require custom register types
   to support re-enabling by such sequence of writes (e.g. some custom
   trigger type may require writing a custom CSR to enable it).
2. OpenOCD may still overwrite these triggers when a user asks to set a
   new WP.

This commit introduces `riscv reserve_trigger ...` command to explicitly
mark the triggers OpenOCD should not touch.

Such approach allows to separate management of custom triggers and
offload it onto the user (e.g. disable/enable such triggers by setting up
an event handler on `step`-related events).

Change-Id: I3339000445185ab221368442a070f412bf44bfab
Signed-off-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>

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