Fix remapping VM attributes on Armada 38x

pmap_remap_vm_attr() function requires indexes to
pte2_attr_tab as the arguments (VM_MEMATTR_).
Mistakenly, instead of them, actual values from the
table were used (PTE2_ATTR_), when applying
work-around for Marvell Armada 38x SoCs.

Submitted by: Marcin Wojtas (mw@semihalf.com)
Reported by: Rafal Kozik (rk@semihalf.com)
Reviewed by: cognet (mentor)
Approved by: cognet (mentor)
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D11704