arm: Allow command line for firmware

When loading a firmware instead of a kernel, we can still pass on any
*user-provided* command line, as /chosen/bootargs is a generic device tree
feature. We just need to make sure to not pass our mangled-for-Linux
version.

This allows to run "firmware" images which make use of a command line,
still are not Linux kernels, like kvm-unit-tests.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>