nds32: define __NDS32_E[BL]__ for sparse

nds32 depends on the macros '__NDS32_E[BL]__' to correctly
select or define endian-specific macros, structures or pieces
of code.

These macros are predefined by the compiler but sparse knows nothing
about them and thus may pre-process files differently from what
GCC would.

Fix this by adding '-D__NDS32_E[BL]__' to CHECKFLAGS.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Acked-by: Greentime Hu <greentime@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>