dt-bindings: rtc: isil,isl1208: Document clock and clock-names properties

dt-bindings: rtc: isil,isl1208: Document clock and clock-names properties

As per the HW manual, XTOSCB bit setting is as follows

If using an external clock signal, set the XTOSCB bit as 1 to
disable the crystal oscillator.

If using an external crystal, the XTOSCB bit needs to be set at 0
to enable the crystal oscillator.

Document clock and clock-names properties.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230623140948.384762-6-biju.das.jz@bp.renesas.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>