ARM: dts: microchip: sama7g5: Fix RTT clock

According to datasheet, Chapter 34. Clock Generator, section 34.2,
Embedded characteristics, source clock for RTT is the TD_SLCK, registered
with ID 1 by the slow clock controller driver. Fix RTT clock.

Fixes: 7540629e ("ARM: dts: at91: add sama7g5 SoC DT and sama7g5-ek")
Link: https://lore.kernel.org/r/20240826165320.3068359-1-claudiu.beznea@tuxon.dev


Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>