ARM: mmp/mmp2: fix cpu_is_mmp2() on mmp2-dt

cpu_is_mmp2() was equivalent to cpu_is_pj4(), wouldn't be correct for
multiplatform kernels. Fix it by also considering mmp_chip_id, as is
done for cpu_is_pxa168() and cpu_is_pxa910() above.

Moreover, it is only available with CONFIG_CPU_MMP2 and thus doesn't work
on DT-based MMP2 machines. Enable it on CONFIG_MACH_MMP2_DT too.

Note: CONFIG_CPU_MMP2 is only used for machines that use board files
instead of DT. It should perhaps be renamed. I'm not doing it now, because
I don't have a better idea.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: stable@vger.kernel.org
Signed-off-by: Olof Johansson <olof@lixom.net>