hw/arm/aspeed: add a 'execute-in-place' property to boot directly from CE0

hw/arm/aspeed: add a 'execute-in-place' property to boot directly from CE0

The overhead for the OpenBMC firmware images using the a custom U-Boot
is around 2 seconds, which is fine, but with a U-Boot from mainline,
it takes an extra 50 seconds or so to reach Linux. A quick survey on
the number of reads performed on the flash memory region gives the
following figures :

  OpenBMC U-Boot      922478 (~ 3.5 MBytes)
  Mainline U-Boot   20569977 (~ 80  MBytes)

QEMU must be trashing the TCG TBs and reloading text very often. Some
addresses are read more than 250.000 times. Until we find a solution
to improve boot time, execution from MMIO is not activated by default.

Setting this option also breaks migration compatibility.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200114103433.30534-5-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>