gpio: sifive: Allow building the driver as a module

This can reduce the kernel image size in multiplatform configurations.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>