Documentation: KVM: Update documentation to indicate KVM is arm64-only

KVM support for 32-bit ARM hosts (KVM/arm) has been removed from the
kernel since commit 541ad015 ("arm: Remove 32bit KVM host
support"). There still exists some remnants of the old architecture in
the KVM documentation.

Remove all traces of 32-bit host support from the documentation. Note
that AArch32 guests are still supported.
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Oliver Upton <oupton@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220308172856.2997250-1-oupton@google.com

Documentation/virt/kvm/devices/vcpu.rst

@@ -108,7 +108,7 @@ using event 0x11 (CPU_CYCLES).
 2. GROUP: KVM_ARM_VCPU_TIMER_CTRL
 =================================
 
-:Architectures: ARM, ARM64
+:Architectures: ARM64
 
 2.1. ATTRIBUTES: KVM_ARM_VCPU_TIMER_IRQ_VTIMER, KVM_ARM_VCPU_TIMER_IRQ_PTIMER
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