image: Add IH_OS_TEE for TEE chain-load boot

This patch adds a new type IH_OS_TEE. This new OS type will be used for
chain-loading to Linux via a TEE.

With this patch in-place you can generate a bootable OPTEE image like this:

mkimage -A arm -T kernel -O tee -C none -d tee.bin uTee.optee

where "tee.bin" is the input binary prefixed with an OPTEE header and
uTee.optee is the output prefixed with a u-boot wrapper header.

This image type "-T kernel -O tee" is differentiated from the existing
IH_TYPE_TEE "-T tee" in that the IH_TYPE is installed by u-boot (flow
control returns to u-boot) whereas for the new IH_OS_TEE control passes to
the OPTEE firmware and the firmware chainloads onto Linux.

Andrew Davis gave the following ASCII diagram:

IH_OS_TEE: (mkimage -T kernel -O tee)
Non-Secure       Secure

                 BootROM
                   |
      -------------
     |
     v
    SPL
     |
     v
   U-Boot ------>
          <-----  OP-TEE
      |
      V
    Linux

IH_TYPE_TEE: (mkimage -T tee)
Non-Secure       Secure

                 BootROM
                   |
      -------------
     |
     v
    SPL ------->
         <-----  OP-TEE
     |
     v
   U-Boot
      |
      V
    Linux

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Suggested-by: Andrew F. Davis <afd@ti.com>
Cc: Harinarayan Bhatta <harinarayan@ti.com>
Cc: Andrew F. Davis <afd@ti.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Kever Yang <kever.yang@rock-chips.com>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Cc: Peng Fan <peng.fan@nxp.com>
Link: http://mrvan.github.io/optee-imx6ul