drivers/fpga: ice40: fix minimum config delay

From FPGA-TN-02001-3.3 "iCE40 Programming and Configuration":

> After driving CRESET_B High or allowing it to float High, the AP must
> wait a minimum of 1200 µs, allowing the iCE40 FPGA to clear its internal
> configuration memory.

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>