spl: add support to booting with OP-TEE

OP-TEE is an open source trusted OS, in armv7, its loading and
running are like this:
loading:
- SPL load both OP-TEE and U-Boot
running:
- SPL run into OP-TEE in secure mode;
- OP-TEE run into U-Boot in non-secure mode;

To make code simple, it would be fine to use IH_OS_TEE for the
os tyle in TPL(just like IH_OS_LINUX is using both in SPL and U-Boot).

Here is the diagram for SPL loading OP-TEE,
IH_OS_TEE:(make u-boot.itb for SPL)
    Non-Secure       Secure

                     BootROM
                       |
                       v
                      SPL
                       |
                       v
          ---------  OP-TEE
         |
         v
       U-Boot
         |
         V
       Linux
For other two king of OP-TEE loading/booting, see commit message:
45b55712d4 image: Add IH_OS_TEE for TEE chain-load boot

More detail:
https://github.com/OP-TEE/optee_os
and search for 'boot arguments' for detail entry parameter in:
core/arch/arm/kernel/generic_entry_a32.S

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Cc: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

include/spl.h

@@ -288,6 +288,19 @@ int spl_mmc_load_image(struct spl_image_info *spl_image,
  */
 void spl_invoke_atf(struct spl_image_info *spl_image);
 
+/**
+ * spl_optee_entry - entry function for optee
+ *
+ * args defind in op-tee project
+ * https://github.com/OP-TEE/optee_os/
+ * core/arch/arm/kernel/generic_entry_a32.S
+ * @arg0: pagestore
+ * @arg1: (ARMv7 standard bootarg #1)
+ * @arg2: device tree address, (ARMv7 standard bootarg #2)
+ * @arg3: non-secure entry address (ARMv7 bootarg #0)
+ */
+void spl_optee_entry(void *arg0, void *arg1, void *arg2, void *arg3);
+
 /**
  * board_return_to_bootrom - allow for boards to continue with the boot ROM
  *