hw/riscv/boot.c: make riscv_find_firmware() static

hw/riscv/boot.c: make riscv_find_firmware() static

The only caller is riscv_find_and_load_firmware(), which is in the same
file.

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>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Message-Id: <20221221182300.307900-5-dbarboza@ventanamicro.com>
Message-Id: <20221229091828.1945072-10-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>