uboot-mvebu: backport patch for Turris Omnia to enable LTO

uboot-mvebu: backport patch for Turris Omnia to enable LTO

Fixes issue while building package uboot-mvebu on OpenWrt 23.05:

u-boot-with-spl.kwb exceeds file size limit:
  limit:  0xf0000 bytes
  actual: 0xf0100 bytes
  excess: 0x100 bytes
make[3]: *** [Makefile:1466: u-boot-with-spl.kwb] Error 1
make[3]: *** Deleting file 'u-boot-with-spl.kwb'
make[3]: Leaving directory '/workspaces/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/u-boot-omnia/u-boot-2024.04'
make[2]: *** [Makefile:83: /workspaces/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/u-boot-omnia/u-boot-2024.04/.built] Error 2

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15307
Signed-off-by: Robert Marko <robimarko@gmail.com>