spi: img-spfi: Limit bit clock to 1/4th of input clock

Although the SPFI BITCLK divider supports a value of up to 255, only
values up to 128 are usable.  This results in a maximum possible bit
clock rate of 1/4th the input clock rate.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>