kconfig/ppc: fix kconfig warning

fix:
arch/ppc/Kconfig:211:warning: type of 'CPU_FREQ_TABLE' redefined from 'tristate' to 'boolean'
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

arch/ppc/Kconfig

@@ -209,7 +209,7 @@ config CPU_FREQ_PMAC
 	  PowerBook.
 
 config CPU_FREQ_TABLE
-	bool
+	tristate
 	depends on CPU_FREQ_PMAC
 	default y