ppc: move at24c to its own CONFIG_ symbol

ppc: move at24c to its own CONFIG_ symbol

AT24c EEPROM is currently gated by CONFIG_I2C, and as such it is
being included in all emulators that use I2C, even if they do not
really need it.  Separate it and, since it was added for the e500
machines, add it to qemu-system-ppc and qemu-system-ppc64.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20180522191743.12872-1-pbonzini@redhat.com>
[lv: rebase]
Signed-off-by: Laurent Vivier <laurent@vivier.eu>