rpi3: update OpenOCD configuration for RPi3

Fixes: https://github.com/OP-TEE/build/issues/257



Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Tested-by: Joakim Bech <joakim.bech@linaro.org> (RPi3+NFS+JTAG)
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>