nds32: Define PLATFORM__SET_BIT for generic_set_bit()

nds32 bitops.h provides a __set_bit() but does not define PLATFORM__SET_BIT
as a result generic_set_bit() is used instead of the architecturally
provided __set_bit().

This patch defines PLATFORM__SET_BIT which means that __set_bit() in nds32
bitops.h will be called whenever generic_set_bit() is called - as opposed
to the default cross-platform generic_set_bit().

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Macpaul Lin <macpaul@andestech.com>

arch/nds32/include/asm/bitops.h

@@ -44,6 +44,8 @@ static inline void __set_bit(int nr, void *addr)
 	*a |= mask;
 }
 
+#define PLATFORM__SET_BIT
+
 extern void clear_bit(int nr, void *addr);
 
 static inline void __clear_bit(int nr, void *addr)