ARM: dts: microchip: sam9x7: fix GMAC clock configuration

The GMAC node incorrectly listed four clocks, including a separate tx_clk
and a TSU GCK clock sourced from ID 67. According to the SAM9X7 clocking
scheme, the GMAC uses only three clocks: HCLK, PCLK, and the TSU GCK
derived from the GMAC peripheral clock (ID 24).

Remove the unused tx_clk, update the clock-names accordingly, and correct
the assigned clock to use GCK 24 instead of GCK 67. This aligns the device
tree with the actual hardware clock topology and prevents misconfiguration
of the GMAC clock tree.

[root@SAM9X75 ~]$ cat /sys/kernel/debug/clk/clk_summary | grep gmac

gmac_gclk      1       1        1        266666666   0          0     50000      Y         f802c000.ethernet           tsu_clk
                                                                                           f802c000.ethernet           tsu_clk
gmac_clk       2       2        0        266666666   0          0     50000      Y         f802c000.ethernet           hclk
                                                                                           f802c000.ethernet           pclk

Fixes: 41af45af8bc3 ("ARM: dts: at91: sam9x7: add device tree for SoC")
Signed-off-by: Mihai Sain <mihai.sain@microchip.com>
Link: https://lore.kernel.org/r/20260309075329.1528-5-mihai.sain@microchip.com
[claudiu.beznea: massaged the patch description]
Signed-off-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>