spi: sh-msiof: Limit minimum divider on R-Car Gen3

On R-Car Gen3 SoCs (excluding R-Car H3 ES1.x, which cannot be used for
SPI due to a hardware erratum), BRPS x BRDV = 1/1 is an invalid divider
setting.
Implement this limitation using an SoC/family-specific minimum divider.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>