Add device nodes for the Timer Units (TMU) on the R-Mobile APE6 SoC,
and the clocks serving them.

Note that TMU channels 1 and 2 are not added, as their interrupts are
not wired to the interrupt controller for the AP-System Core (INTC-SYS),
only to the interrupt controller for the AP-Realtime Core (INTC-RT).
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/1a60832f3ba37afb4a5791f4e5db4610ab31beb3.1710864964.git.geert+renesas@glider.be