clk: renesas: r8a7778: Remove struct r8a7778_cpg

All but the data member of the r8a7778_cpg structure are unused, so the
whole structure can be replaced by the single member used.

Remove the mapping of the CPG registers, as no code uses the mapped
registers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/4123c1c40a901195f00a825d434553d2052829aa.1654694831.git.geert+renesas@glider.be