target/riscv: add PRIV_VERSION_LATEST

All these generic CPUs are using the latest priv available, at this
moment PRIV_VERSION_1_12_0:

- riscv_any_cpu_init()
- rv32_base_cpu_init()
- rv64_base_cpu_init()
- rv128_base_cpu_init()

Create a new PRIV_VERSION_LATEST enum and use it in those cases. I'll
make it easier to update everything at once when a new priv version is
available.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230517135714.211809-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.h

@@ -61,6 +61,8 @@ enum {
     PRIV_VERSION_1_10_0 = 0,
     PRIV_VERSION_1_11_0,
     PRIV_VERSION_1_12_0,
+
+    PRIV_VERSION_LATEST = PRIV_VERSION_1_12_0,
 };
 
 #define VEXT_VERSION_1_00_0 0x00010000