spi: rspi: Clean up Bit Rate Division Setting handling

Add a macro for configuring the Bit Rate Division Setting field in
Command Registers, instead of open-coding the same operation using a
hardcoded shift.
Rename "div" to "brdv", as it is not a plain divider value, but controls
a power-of-two divider.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819125904.20938-3-geert+renesas@glider.be


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