spi: rspi: Increase bit rate range for QSPI

Increase bit rate range for QSPI by extending the range of supported
dividers:
  1. QSPI supports a divider of 1, by setting SPBR to zero, increasing
     the upper limit from 48.75 to 97.5 MHz on R-Car Gen2,
  2. Make use of the Bit Rate Frequency Division Setting field in
     Command Registers, to decrease the lower limit from 191 to 24 kbps
     on R-Car Gen2.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-6-geert+renesas@glider.be


Signed-off-by: Mark Brown <broonie@kernel.org>