nios2: Define PLATFORM__CLEAR_BIT for generic_clear_bit()

nios2 bitops.h provides a __clear_bit() but does not define
PLATFORM__CLEAR_BIT as a result generic_clear_bit() is used instead of the
architecturally provided __clear_bit().

This patch defines PLATFORM__CLEAR_BIT which means that __clear_bit() in
nios2 bitops.h will be called whenever generic_clear_bit() is called - as
opposed to the default cross-platform generic_clear_bit().

Signed-off-by: Bryan O'Donoghue <pure.logic@nexus-software.ie>
Cc: Thomas Chou <thomas@wytron.com.tw>

arch/nios2/include/asm/bitops/non-atomic.h

@@ -30,6 +30,8 @@ static inline void __clear_bit(int nr, volatile unsigned long *addr)
 	*p &= ~mask;
 }
 
+#define PLATFORM__CLEAR_BIT
+
 /**
  * __change_bit - Toggle a bit in memory
  * @nr: the bit to change