powerpc: Remove CONFIG_POWER4

Although the name CONFIG_POWER4 suggests that it controls support for
power4 cpus, this symbol is actually misnamed.

It is a historical wart from the powermac code, which used to support
building a 32-bit kernel for power4. CONFIG_POWER4 was used in that
context to guard code that was 64-bit only.

In the powermac code we can just use CONFIG_PPC64 instead, and in other
places it is a synonym for CONFIG_PPC_BOOK3S_64.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>