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