aspeed: Add boot stub for smp booting

aspeed: Add boot stub for smp booting

This is a boot stub that is similar to the code u-boot runs, allowing
the kernel to boot the secondary CPU.

u-boot works as follows:

 1. Initialises the SMP mailbox area in the SCU at 0x1e6e2180 with default values

 2. Copies a stub named 'mailbox_insn' from flash to the SCU, just above the
    mailbox area

 3. Sets AST_SMP_MBOX_FIELD_READY to a magic value to indicate the
    secondary can begin execution from the stub

 4. The stub waits until the AST_SMP_MBOX_FIELD_GOSIGN register is set to
    a magic value

 5. Jumps to the address in AST_SMP_MBOX_FIELD_ENTRY, starting Linux

Linux indicates it is ready by writing the address of its entrypoint
function to AST_SMP_MBOX_FIELD_ENTRY and the 'go' magic number to
AST_SMP_MBOX_FIELD_GOSIGN. The secondary CPU sees this at step 4 and
breaks out of it's loop.

To be compatible, a fixed qemu stub is loaded into the mailbox area. As
qemu can ensure the stub is loaded before execution starts, we do not
need to emulate the AST_SMP_MBOX_FIELD_READY behaviour of u-boot. The
secondary CPU's program counter points to the beginning of the stub,
allowing qemu to start secondaries at step four.

Reboot behaviour is preserved by resetting AST_SMP_MBOX_FIELD_GOSIGN
when the secondaries are reset.

This is only configured when the system is booted with -kernel and qemu
does not execute u-boot first.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>