drivers: fpga: simplify load mode selection of iCE40

Replace the enum for load modes for the iCE40 with a boolean flag,
as there are only two options:
- SPI: default, which should be used whenever possible
- GPIO bitbang: workarorund, in case a low-end microcontroller is used

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>