Add a OP-TEE early loading function which expects a pointer to a valid
tee binary and the device tree. OP-TEE will then be started and barebox
will continue to run in normal mode.

The function start_optee_early should be used in a boards lowlevel.c
file. Ensure that barebox has been relocated and a proper c environment
has been setup beforehand. Depending on the OP-TEE configuration, the
fdt will be modified. If the internal barebox device tree is passed,
OP-TEE will overwrite barebox PBL memory during this modification. Copy
the fdt to a save memory location beforehand to avoid a corruption of
barebox PBL memory.

This also moves the OP-TEE Kconfig symbols into a separate menu.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>