ANDROID: KVM: arm64: Always unmap the pvmfw region at stage-2

The donation of the pvmfw region to pKVM is currently done transparently
as part of fix_host_ownership(). However, this function only runs over PA
ranges covered by the memblock list, although there is no guarantee for
the pvmfw region to be advertised in a memory node in DT. In this case,
the pKVM init will appear to succeed while silently keeping valid host
stage-2 mappings to the pvmfw region.

Fix this by forcefully registering the pvmfw region in the pKVM memblock
list.

Bug: 357781595
Bug: 424382332
Reported-by: Bartłomiej Grzesik <bgrzesik@google.com>
Suggested-by: Will Deacon <willdeacon@google.com>
Change-Id: I8f5498df25debb432b7dffd1e40a8910bcec7b49
Signed-off-by: Quentin Perret <qperret@google.com>