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

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

All generic CPUs call riscv_cpu_add_user_properties(). The 'max' CPU
calls riscv_init_max_cpu_extensions(). Both can be moved to a common
instance_post_init() callback, implemented in riscv_cpu_post_init(),
called by all CPUs. The call order then becomes:

riscv_cpu_init() -> cpu_init() of each CPU -> .instance_post_init()

In the near future riscv_cpu_post_init() will call the init() function
of the current accelerator, providing a hook for KVM and TCG accel
classes to change the init() process of the CPU.

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: <20230925175709.35696-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>