KVM: arm64: Declutter host PSCI 0.1 handling

Although there is nothing wrong with the current host PSCI relay
implementation, we can clean it up and remove some of the helpers
that do not improve the overall readability of the legacy PSCI 0.1
handling.

Opportunity is taken to turn the bitmap into a set of booleans,
and creative use of preprocessor macros make init and check
more concise/readable.

Suggested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>