efi_loader: implement EFI_DT_FIXUP_PROTOCOL

A boot manager like GRUB can use the protocol to

* apply U-Boot's fix-ups to the a device-tree
* let U-Boot make memory reservations according to the device-tree
* install the device-tree as a configuration table

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>