Analyse XIP performance issues

[legoater]

With the 'execute-in-place' patch to boot directly from CE0, we see a large
impact on boot time.

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.