clk: at91: sam9x7: Rename macb0_clk to gmac_clk

Update the peripheral clock name for ID 24 from macb0_clk to gmac_clk to
match the actual GMAC hardware block present on SAM9X7 SoCs and the
datasheet description.

Signed-off-by: Mihai Sain <mihai.sain@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://lore.kernel.org/r/20260309075329.1528-3-mihai.sain@microchip.com
[claudiu.beznea: massaged the patch description]
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>