eco32: pgtable: make PTRS_PER_PTE accesible from asm

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: pgtable: set _PAGE_PRESENT = _PAGE_VALID

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>