hw/riscv: implement CBQRI capacity controller

Implement a capacity controller according to the Capacity and Bandwidth
QoS Register Interface (CBQRI) which supports these capabilities:

  - Number of access types: 2 (code and data)
  - Usage monitoring operations: CONFIG_EVENT, READ_COUNTER
  - Event IDs supported: None, Occupancy
  - Capacity allocation ops: CONFIG_LIMIT, READ_LIMIT, FLUSH_RCID

Link: https://github.com/riscv-non-isa/riscv-cbqri/blob/main/riscv-cbqri.pdf


Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>