eco32: process: remove BUG() todo marker

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: add incomplete ret_from_syscall

As the title says, incomplete, but what's actually there should
be about correct.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: head: check present bit in pte

Generate a page fault if the present bit isn't set.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
eco32: head: write to random tlb in miss handler

The tbwr instruction will generate a random tlb index and
write what is present in TLB_ENTRY_LOW and TLB_ENTRY_HIGH to
that.

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