T-Head implemented a page attribute extension that violates the RISC-V

T-Head implemented a page attribute extension that violates the RISC-V
specification.  The default attributes result in memory being uncached
which makes the system perform like a slug.  So implement a workaround
that is designed to make implementation of the Svpbmt extension that is
part of the latest published RISC-V specification.  This gets us a bit
further booting OpenBSD on an Allwinner D1 SoC.

ok mlarkin@, jca@