target/riscv/cpu.c: add riscv_bare_cpu_init()

target/riscv/cpu.c: add riscv_bare_cpu_init()

Next patch will add more bare CPUs. Their cpu_init() functions would be
glorified copy/pastes of rv64i_bare_cpu_init(), differing only by a
riscv_cpu_set_misa() call.

Add a new .instance_init for the TYPE_RISCV_BARE_CPU typ to avoid this
code repetition. While we're at it, add a better explanation on why
we're disabling the timing extensions for bare CPUs.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240122123348.973288-2-dbarboza@ventanamicro.com>
[ Changes by AF:
 - Rebase on latest changes
]
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>