KVM: arm64: Change CONFIG_KVM to a menuconfig entry

Changing CONFIG_KVM to be a 'menuconfig' entry in Kconfig mean that we
can straightforwardly enumerate optional features, such as the virtual
PMU device as dependent options.

Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200505154520.194120-4-tabba@google.com