xtensa port: Fixed crash on startup on CP enabled cores

The issue was that cpStack was changed to a memory buffer by commit
https://gerrit.zephyrproject.org/r/#/c/12816


However the assembly code was expecting it to be a pointer and thus
issuing an indirection, that leads to wrong addresses.

The fix removed this unnecessary indirection and thus the inherent
invalid memory access exception.

Issue: ZEP-1997
Change-Id: I843f049212f2d116a01b05367a284209f463a5e7
Signed-off-by: Mazen NEIFER <mazen@nestwave.com>