MX31: removed warning due to missing prototype

Drop warning caused by missing prototype for
mxc_hw_watchdog_reset().

Signed-off-by: Stefano Babic <sbabic@denx.de>

arch/arm/include/asm/arch-mx31/clock.h

@@ -31,5 +31,6 @@ extern void mx31_set_pad(enum iomux_pins pin, u32 config);
 
 void mx31_uart1_hw_init(void);
 void mx31_spi2_hw_init(void);
+void mxc_hw_watchdog_enable(void);
 
 #endif /* __ASM_ARCH_CLOCK_H */