board: iot2050: update build documentation for OP-TEE

board: iot2050: update build documentation for OP-TEE

Set ta-target explicitly to correspond with OP-TEE recipe in
siemens/meta-iot2050.

Errors without explicit set of ta-target:
aarch64-linux-gnu-gcc: error: unrecognized command-line option ‘-mthumb’
aarch64-linux-gnu-gcc: error: unrecognized command-line option ‘-mno-unaligned-access’
aarch64-linux-gnu-gcc: error: unrecognized command-line option ‘-mfloat-abi=hard’
make: *** [mk/compile.mk:159: out/arm-plat-k3/ta_arm32-lib/libdl/dlfcn.o] Error 1

Signed-off-by: Ivan Mikhaylov <ivan.mikhaylov@siemens.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>