Add general purpose timer GPTMR

[stnolting]

This PR adds a general purpose 32-bit timer module GPTMR (feature request). The timer provides three memory-mapped registers CTRL (control and status), THRES (counter threshold) and COUNT (actual counter). It can operate either in single-shot mode (COUNT increments until matching THRES once) or continuous mode (COUNT increments until matching THRES, reset and restarts). Whenever COUNT matches THRES an interrupt request is generated. The timer increment can be controlled by selecting one out of eight clock pre-scalers.

The GPTMR is mapped to the previously unused base address 0xFFFFFF60 and can be enabled for synthesis by the new IO_GPTMR_EN generic (default = false). It uses FIRQ 12 to send interrupt requests to the CPU.

New design files:

• rtl/core/neorv32_gptmr.vhd
• sw/lib/include/neorv32_gptmr.h
• sw/lib/source/neorv32_gptmr.c