spi: rspi: Increase accuracy of bit rate for RZ

When you leave the clock divider at 0, 130kHz is the lowest you can go.
Also, by adjusting the clock divider you can get more accurate resolutions
for clock speeds lower than 16MHz. This patch uses the clock divider as
part of the bit rate setup.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>