hw/ppc: e500: Use a macro for the platform clock frequency

At present the platform clock frequency is using a magic number.
Convert it to a macro and use it everywhere.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

Message-Id: <1612362288-22216-1-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>