KVM: arm64: Rework logic to en/decode VTCR_EL2.{SL0, SL2} fields

In order to support 5 level translation, FEAT_LPA2 introduces the 1-bit
SL2 field within VTCR_EL2 to extend the existing 2-bit SL0 field. The
SL2[0]:SL0[1:0] encodings have no simple algorithmic relationship to the
start levels they represent (that I can find, at least), so replace the
existing macros with functions that do lookups to encode and decode the
values. These new functions no longer make hardcoded assumptions about
the maximum level and instead rely on KVM_PGTABLE_FIRST_LEVEL and
KVM_PGTABLE_LAST_LEVEL.

This is preparatory work for enabling 52-bit IPA for 4KB and 16KB pages
with FEAT_LPA2.

No functional change intended.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>