configs/freescale_imx91frdm: bump U-Boot, TF-A and Linux

Since Buildroot commit [1] "package/gcc: switch to GCC 15.x as the
default", compiling freescale_imx91frdm_defconfig fails in TF-A with
error:

    include/drivers/nxp/trdc/imx_trdc.h:7: error: header guard 'IMX_TRDC_H' followed by '#define' of a different macro [-Werror=header-guard]
        7 | #ifndef IMX_TRDC_H
    include/drivers/nxp/trdc/imx_trdc.h:8: note: 'IMX_XRDC_H' is defined here; did you mean 'IMX_TRDC_H'?
        8 | #define IMX_XRDC_H

The issue happen because this defconfig is using a Buildroot
(internal) toolchain which uses gcc version 15 by default.

This issue was fixed upstream in [2], first included in TF-A v2.13.

This commit updates U-Boot, TF-A and Linux to use the NXP
tag lf-6.18.20-2.0.0, which include this fixes.

Note that, as described in the commit log of [3] which was used as a
base for this defconfig, U-Boot and Linux were forks. Now the
necessary patches are included in the vendor BSP, this commit
switches those repos to use the vendor ones.

This commit also removes the linux-headers.hash custom hash file
which was never needed, because the defconfig used an external
toolchain.

Also, due to the upstream uboot commit [4], this commit adds a uboot
config fragment to disable CONFIG_EFI_CAPSULE_AUTHENTICATE, to
remove the dependency on the command "cert-to-efi-sig-list", provided
the host efitools package, which is not yet in Buildroot.

This commit was run tested on board 700-94610 Rev A1.

[1] a75b8623f5
[2] 2e9198d0e0%5E%21/
[3] 2e05f22ff8
[4] fd58c275f6

Signed-off-by: Julien Olivain <ju.o@free.fr>

board/freescale/imx91frdm/patches/linux/linux.hash

@@ -1,2 +1,2 @@
 # Locally calculated
-sha256  3f5017e85ba6c490be597670033a7eaba14e3c34af4e53d24dc3762c3deece83  linux-imx-lf-6.6.36-2.1.0-imx91frdm.tar.gz
+sha256  a8f55764fd1418db8adfd2711c0abb7d5464a8ee1cbfd0f6878f1059ecfa1316  linux-imx-lf-6.18.20-2.0.0.tar.gz