firmware: psci: Do not bind driver if U-Boot runs in EL3

There is no reason to bind psci driver if U-Boot runs in EL3 because
SMC/HVC instructions can't be called. That's why detect this state and
don't let user to crash from prompt by performing reset or poweroff
commands (if enabled).

Signed-off-by: Michal Simek <michal.simek@xilinx.com>