powerpc: define get_cycles macro for arch-override

PowerPC defines a get_cycles() function, but it does not do the usual
`#define get_cycles get_cycles` dance, making it impossible for generic
code to see if an arch-specific function was defined. While the
get_cycles() ifdef is not currently used, the following timekeeping
patch in this series will depend on the macro existing (or not existing)
when defining random_get_entropy().

Cc: Thomas Gleixner <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Acked-by: Michael Ellerman <[e-mail]>
Signed-off-by: Jason A. Donenfeld <[e-mail]>

arch/powerpc/include/asm/timex.h

···
 {
 	return mftb();
 }
+#define get_cycles get_cycles
 
 #endif	/* __KERNEL__ */
 #endif	/* _ASM_POWERPC_TIMEX_H */