target/riscv: create TYPE_RISCV_VENDOR_CPU

target/riscv: create TYPE_RISCV_VENDOR_CPU

We want to add a new CPU type for bare CPUs that will inherit specific
traits of the 2 existing types:

- it will allow for extensions to be enabled/disabled, like generic
  CPUs;

- it will NOT inherit defaults, like vendor CPUs.

We can make this conditions met by adding an explicit type for the
existing vendor CPUs and change the existing logic to not imply that
"not generic" means vendor CPUs.

Let's add the "vendor" CPU type first.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218125334.37184-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>