soc: xtensa/dc233c: remove xtensa_dc233c_stack_ptr_is_sane

The generic stack pointer checker in the architecture code is
enough so we can remove the platform specific one. Besides,
xtensa_dc233c_stack_ptr_is_sane() does not do much checking
either.

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