KVM: arm64: Clean up kvm makefiles

Consolidate references to the CONFIG_KVM configuration item to encompass
entire folders rather than per line.

Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200505154520.194120-5-tabba@google.com