dt-bindings: clock: versaclock3: Add description for #clock-cells property

Add description for "#clock-cells" property to map indexes to the clock
output in the Table 3. ("Output Source") in the 5P35023 datasheet
(ie: {REF,SE1,SE2,SE3,DIFF1,DIFF2}. Also update the "assigned-clock-rates"
in the example.

While at it, replace clocks phandle in the example from x1_x2->x1 as
X2 is a different 32768 kHz crystal.

Suggested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230824104812.147775-2-biju.das.jz@bp.renesas.com
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>