txmmu/rxmmu: Need valid bit in page table entries

[olajep]

After I switched RX path to MMU from static remap, I started to see sporadic external fetch abort errors when running the mailbox test. (The only test that requires reading from the FPGA elink regs)
I think one error source was a flaky VDD_DSP so I took a step back and left the regulator always on, but there were still issues.
I've been all over the place but in the end I think it boils down to this one stupid line in the mailbox example. DOH!
Running exhaust test now, knock on wood.

e_open(&dev, 0, 0, platform.rows, platform.cols);
e_load_group("emain.elf", &dev, 0, 0, 1, 1, E_FALSE);
e_start_group(&dev);

e_open(&dev, 0, 0, 1, 1);
e_load_group("emain.elf", &dev, 0, 0, 1, 1, E_FALSE);
e_start_group(&dev);

So I loaded the program to core (0,0), but started all cores. Random SRAM data == random instructions == random off-chip access to an invalid address that causes the AXI slave to time out.
This wasn't visible with static remapping since all accesses was forced to [0x8e000000-0x90000000] by the mask.

At any rate, an invalid off-chip access from Epiphany should never bring down the entire system. It's an easy programming error, even I can do it ;)
("External Fetch Abort" in kernel when locks are held requires reboot)

// Ola