KVM: arm64: pkvm: Use direct function pointers for cpu_{on,resume}

KVM: arm64: pkvm: Use direct function pointers for cpu_{on,resume}

Instead of using a boolean to decide whether a CPU is booting or
resuming, just pass an actual function pointer around.

This makes the code a bit more straightforward to understand.

Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260321212419.2803972-5-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>