[riscv] Support the standard Svpbmt extension for page-based memory types

[riscv] Support the standard Svpbmt extension for page-based memory types

Set the appropriate Svpbmt type bits within page table entries if the
extension is supported.  Tested only in QEMU so far, due to the lack
of availability of real hardware supporting Svpbmt.

Signed-off-by: Michael Brown <mcb30@ipxe.org>