arch: arm64: Add the sp variable in _esf_t

As the preparation for enabling safe exception stack, add a variable in
_esf_t to save the user stack held by sp_el0 at the point of the
exception happening from EL0. The newly added variable in _esf_t is
named sp from which the user stack will be restored when exceptions eret
to EL0.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>