arm64: zynqmp: Change firmware dependency

In case of mini U-Boot configurations there is no need to enable firmware
driver which just consume space for nothing. That's why add an option to
disable it.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/d439399160ff3374f2b39f54f7dd70fa8c8bfea0.1642162121.git.michal.simek@xilinx.com