[ARM] pxa/cpufreq: fix index assignments for end marker

I stumbled over two small things regarding the .index field assignment
in the dynamically created cpu frequency tables for pxa2xx and pxa3xx.

Even though that doesn't currently cause any problem, it should still be
fixed in case the logic in the CPUFREQ core changes.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>