ARM: dts: renesas: r8a73a4: Add cp clock

ARM: dts: renesas: r8a73a4: Add cp clock

Add the Common Peripheral (CP) clock, which is driven by the main
clock / 2 during normal system operation, but may be driven by EXTALR
during early system boot, when SYSCLK_EN is still low.  As the latter is
irrelevant to Linux, just model it as a fixed clock driven from
main_div2_clk.

Switch all users of main_div2_clk that are documented to be clocked by
the CP clock to cp_clk, to better reflect the actual clock topology.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/f9826b0755207a1e16871c17daca109fb11f3868.1705315614.git.geert+renesas@glider.be