Add support for starting TFA from U-Boot running in EL3 as part of
fitImage boot, so the user can start U-Boot in the highest privilege
level on the platform, bundle TFA, Linux, DT into a single fitImage
and boot such a bundle as a whole.

There are two main benefits of this approach. First is the ability
to run U-Boot in EL3, where it has unrestricted access to the entire
system and can act as a useful debug tool, as it was always intended
to be used. Second is the ability to easily and safely update of any
component in the fitImage, be it TFA, Linux or DT.

The boot process is similar to regular Linux with DT fitImage boot
process, except the TFA has to be bundled into the fitImage. For the
bundling instructions, see below. The TFA is started as a 'loadables'
with custom U_BOOT_FIT_LOADABLE_HANDLER and armv8_switch_to_el2_prep()
handling implemented in board code, and performing the handoff and
boot in case the TFA was loaded.

The loadables handler is optional and meant to set up any sort of
handoff structures used by the TFA BL31 or perform any other setup
that is needed by the blob. The custom armv8_switch_to_el2_prep()
has to implement the jump to TFA BL31 with return to U-Boot just
before booting the Linux kernel.

Example fitImage image and configuration section:

/dts-v1/;

/ {
    description = "Linux kernel with FDT blob and TFA BL31";

    images {
        kernel-1 { ... };
        fdt-1 { ... };
        atf-1 {                  /* This is the TFA BL31 image */
            description = "TFA BL31";
            data = /incbin/("../build/plat/release/bl31.bin");
            type = "tfa-bl31";
            arch = "arm64";
            os = "arm-trusted-firmware";
            compression = "none";
            load = <0x46400000>;
            entry = <0x46400000>;
        };
    };

    configurations {
        default = "conf-1";
        conf-1 {
            description = "Boot Linux";
            kernel = "kernel-1";
            fdt = "fdt-1";
            loadables = "atf-1"; /* This is the TFA BL31 loadable */
        };
    };
};

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>