T-Head cpuinfo

This is a simple CPU information decoder for T-Head cores.

Detailed information is not available regarding model etc. (CPUINFO0).

Per the C906 manual:

The machine mode processor model register (MCPUID) stores the processor model information. Its reset value is determined by the product itself and complies with the Pingtouge product definition specifications to facilitate software identification. By continuously reading the MCPUID register, up to 7 different return values can be obtained to represent C906 product information, as shown in Figure ??.

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To read out the register, RISC-V CSR 0xfc0:

/// T-Head CPU model register
fn print_cpuid() {
    let mut id: u32;
    for i in 0..7 {
        unsafe { asm!("csrr {}, 0xfc0", out(reg) id) };
        println!("MCPUID {i}: {id:08x}");
    }
}