powerpc/powermac: add missing g5_phy_disable_cpu1() declaration

g5_phy_disable_cpu1() is used outside of platforms/powermac/feature.c,
so it should have a declaration to fix W=1 warning:

  arch/powerpc/platforms/powermac/feature.c:1533:6:
    error: no previous prototype for ‘g5_phy_disable_cpu1’ [-Werror=missing-prototypes]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210924105653.46963-1-krzysztof.kozlowski@canonical.com

arch/powerpc/platforms/powermac/pmac.h

@@ -16,6 +16,8 @@ struct rtc_time;
 
 extern int pmac_newworld;
 
+void g5_phy_disable_cpu1(void);
+
 extern long pmac_time_init(void);
 extern time64_t pmac_get_boot_time(void);
 extern void pmac_get_rtc_time(struct rtc_time *);

arch/powerpc/platforms/powermac/smp.c

@@ -875,8 +875,6 @@ static int smp_core99_cpu_online(unsigned int cpu)
 
 static void __init smp_core99_bringup_done(void)
 {
-	extern void __init g5_phy_disable_cpu1(void);
-
 	/* Close i2c bus if it was used for tb sync */
 	if (pmac_tb_clock_chip_host)
 		pmac_i2c_close(pmac_tb_clock_chip_host);