siemens: capricorn: set max-frequency for usdhc1

This is required since

commit aebb523a23 ("mmc: mmc-uclass: Use max-frequency from device tree with default handling")

and the related patches of the same series.

The error observed without this change is:
  Autobooting in 3 seconds, press "<Esc><Esc>" to stop
  EXT_CSD[179], PARTITION_CONFIG:
  BOOT_ACK: 0x0
  BOOT_PARTITION_ENABLE: 0x1 (boot0)
  PARTITION_ACCESS: 0x0 (user)
  Loading from eMMC ...fit

  U-Boot SPL 2026.01-4238dcfcbfe (Jan 09 2026 - 08:19:45 +0000)

For this example it's the following commands which does no longer work
for larger images:

  ext4load mmc 0:1 0x88000000 boot/fitImage

On latest master branch the problematic commit gets reverted with

commit c4f5b1d4b0 ("Revert "mmc: mmc-uclass: Use max-frequency from device tree with default handling"")

but for v2026.01 this fix is still required. Maybe it's anyway a good
idea to have this property set explicitly.

Signed-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
Signed-off-by: Heiko Schocher <hs@nabladev.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>

arch/arm/dts/imx8-capricorn.dtsi

@@ -125,6 +125,7 @@
 &usdhc1 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&pinctrl_usdhc1>;
+	max-frequency = <52000000>;
 	clock-frequency=<52000000>;
 	no-1-8-v;
 	bus-width = <8>;