xtensa: remove xtensa-asm2.h

xtensa-asm2.h only contains the function declaration of
xtensa_init_stack() which is only used in one file. So
make the actual implementation a static function in that
file. Also there is really no need to expose stack init
function as arch public API. So remove xtensa-asm2.h.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>