board: Add PowerPC mac99

PowerPC kernel developers were after a userspace for testing 32-bit
powerpc kernels. This machine both suits that requirement and has
support in qemu. It's also a fairly common piece of 32-bit ppc hardware.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Tested-by: Daniel Axtens <dja@axtens.net>
[Peter: lock kernel/headers to 5.2.4]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>