hw/riscv/virt.c: add 'base' arg in create_fw_cfg()

hw/riscv/virt.c: add 'base' arg in create_fw_cfg()

The function can receive the value via s->memmap[VIRT_FW_CFG].base from
the caller, avoiding the use of virt_memmap.

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250429125811.224803-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>