⚠️ [rtl] cleanup CPU standard counters, remove CPU_CNT_WIDTH generic

[stnolting]

This PR is another cleanup of the CPU's standard counters.

⚠️ The CPU_CNT_WIDTH generic is removed. Hence, the width of the standard CPU counters ([m]cycle and [m]instret) is always 64-bit (low-word + high-word).

For area-constrained setups the standard CPU counters can be excluded from synthesis by disabling the Zicntr ISA extension (via the CPU_EXTENSION_RISCV_Zicntr generic).