arm: move kernel loading into arm/kvm.c

For some reasons (probably to have easy access to the command line)
the kernel loading for arm and arm64 was located in arm/fdt.c.
Move the routines to kvm.c (where other architectures put it) to
only have real device tree code in fdt.c. We use the pointer in
struct kvm to access the command line string.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>