ANDROID: KVM: arm64: Fix KVM_HOST_S2_DEFAULT_MMIO_PTE encoding

Commit 2ab1955d ("ANDROID: KVM: arm64: Allow setting {P,U}XN in
stage-2 PTEs") changed KVM_PTE_LEAF_ATTR_HI_S2_XN to refer to the XN
field of the pte (i.e. bits 54:53) instead of just bit 54 and forgot
to update KVM_HOST_S2_DEFAULT_MMIO_PTE in the process.

Consequently, coalescing will fail to apply to default MMIO ptes and
may inadvertently trigger for ptes marked as UXN. Fix the mask.

Cc: Quentin Perret <qperret@google.com>
Cc: Sebastian Ene <sebastianene@google.com>
Bug: 308395502
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I4f85eae7ccee008a1161e09739c1fd4f3e1b6e2b