dt-bindings: clock: renesas,r9a08g045-vbattb: Document VBATTB

dt-bindings: clock: renesas,r9a08g045-vbattb: Document VBATTB

The VBATTB IP of the Renesas RZ/G3S SoC controls the clock for RTC,
the tamper detector and a small general usage memory of 128B.

The VBATTB controller controls the clock for the RTC on the Renesas
RZ/G3S. The HW block diagram for the clock logic is as follows:

           +----------+ XC   `\
RTXIN  --->|          |----->| \       +----+  VBATTCLK
           | 32K clock|      |  |----->|gate|----------->
           | osc      | XBYP |  |      +----+
RTXOUT --->|          |----->| /
           +----------+      ,/

One could connect as input to this HW block either a crystal or
an external clock device. This is board specific.

After discussions w/ Stephen Boyd the clock tree associated with this
hardware block was exported in Linux as:

input-xtal
  xbyp
  xc
     mux
        vbattclk

where:
- input-xtal is the input clock (connected to RTXIN, RTXOUT pins)
- xc, xbyp are mux inputs
- mux is the internal mux
- vbattclk is the gate clock that feeds in the end the RTC

to allow selecting the input of the MUX though assigned-clock DT
properties, using the already existing clock drivers and avoid adding
other DT properties.

This allows select the input of the mux based on the type of the
connected input clock:
- if the 32768 crystal is connected as input for the VBATTB,
  the input of the mux should be xc
- if an external clock device is connected as input for the VBATTB the
  input of the mux should be xbyp

Add bindings for the VBATTB controller.

Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Link: https://lore.kernel.org/20241101095720.2247815-2-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>