KVM: selftests: arm64: Rework TCR_EL1 configuration

The current way we initialise TCR_EL1 is a bit cumbersome, as
we mix setting TG0 and IPS in the same swtch statement.

Split it into two statements (one for the base granule size, and
another for the IPA size), allowing new modes to be added in a
more elegant way.

No functional change intended.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Link: https://lore.kernel.org/r/20211227124809.1335409-5-maz@kernel.org