dt-bindings: clk: at91: Add clock IDs for the slow clock controller

Add clock IDs for the slow clock controller. Previously, raw numbers
were used (0 or 1) for clocks generated by the slow clock controller. This
leads to confusion and wrong IDs were used on few device trees. To avoid
this add macros.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240826173116.3628337-2-claudiu.beznea@tuxon.dev
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>