Revert "mboot64: maybe fix the AEM timer."

This reverts commit 54603fe8fb4026b1ad45042eda6c3d1136b8b971.

Don't bother as the workaround way doesn't really work as it's
unimplemented on the AEM just as it is unimplemented on the FM.
mboot64: Initialize el3 vbar + stack and clean up exception logic.

There were dumb bugs in exception code (Which is why I shouldn't code
at 4 AM - I'd fetch the wrong ELR/SPSR based on previous mode
instead of current). Also, made sure that exc.c now works
correctly both in EL3 and EL2.

The EL3 handling will become an SMC handler and a small debugger
eventually.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>