target/riscv: rename ext_icboz to ext_zicboz

target/riscv: rename ext_icboz to ext_zicboz

Add a leading 'z' to improve grepping. When one wants to search for uses
of zicboz they're more likely to do 'grep -i zicboz' than 'grep -i
icboz'.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Message-ID: <20231012164604.398496-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>