hw/ppc/meson: Allow e500 boards to be enabled separately

Gives users more fine-grained control over what should be compiled into
QEMU.

Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20221003203142.24355-2-shentey@gmail.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>