clk: renesas: rcar-gen4: Use FIELD_GET()

Improve readability by using the FIELD_GET() helper instead of
open-coding the same operation, and by adding field definitions to get
rid of hardcoded values.

While at it, move register definitions that are only used inside the
rcar-gen4-cpg.c source file out of the rcar-gen4-cpg.h header file.
Add a "CPG_" prefix to SD0CKCR1.  Add comments where appropriate.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/fb19ad829738f02effa340fa04c178a162d41202.1721648548.git.geert+renesas@glider.be