arm_64 exception vector and el2 stack

[Michael Grunditz]

> Running out of ideas.

I see now , after upgrading sourcetree to the latest release, that MMU
 is disabled in ctr0.s. Doing that made Genode::raw stop working and
no difference in EL switching. I am thinking.. I tried to disable
dcache in uboot before.. which should turn mmu off. But since it now
is disabled in genode startup I can't see that necessary.

I would like to see a bit more ARM init in Genode /base-hw . Would be
nice to turn everything off and then bring in the wanted
functionality. That would make a common ground. In my case there are a
bunch of blobs running before uboot. I don't have control over them at
all. It can boot linux, but linux does a complete setup of cpu last
time I looked. u-boot runs in el2, caches and mmu is on.

The blobs are the only difference from what I did on rk3399, besides
different ARM core.

......
Michael