Document APB Timer's use as MTIME

Placeholder Mike Thompson
2026-02-12T15:40:03Z

One of the use-cases of the APB Timer is to implement the MTIME platform CSR. This is hinted at, but not stated explicitly in the User Manual. This issue can be considered resolved when:
- the ABP Timer chapter of the User Manual explicitly states that one of the intended use-cases of the block is to implement MTIME.
- the [Firmware Guidelines](https://docs.openhwgroup.org/projects/core-v-mcu/doc-src/ip-blocks/apb_timer.html#firmware-guidelines) includes a sub-section illustrating how to program the APB Timer to implement MTIME.