Intel DMAR: remove parsing of 6-level paging capability

Early versions of the VT-d spec mentioned 6-level paging support as a
possible value for the SAGAW capability, but later versions removed it
and SAGAW=0x10 is currently listed as a reserved value.

The 6-level (agaw=64) entry in sagaw_bits is furthermore problematic
with clang15 because the attempted comparison against 1ULL << 64 in
dmar_maxaddr2mgaw() causes the compiler to elide the last iteration
of the initial loop, which bypasses the subsequent logic to find the
greatest HW-supported address width. This results in 5-level paging
always being selected regardless of whether the hardware supports it,
which can result address translation failure due to invalid context-
entry programming.

Reviewed by: kib
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D39896