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