spi: dw: Clamp the minimum clock speed

The DW SPI interface has a 16-bit clock divider, where the bottom bit
of the divisor must be 0. Limit how low the clock speed can go to
prevent the clock divider from being truncated, as that could lead to
a much higher clock rate than requested.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>