PM / devfreq: memlat: Increase range of ratio_ceil tunables

Some instances of memlat governor need to use a higher value for ratio_ceil
to get a better power/performance balance. So increase the max limit from
10000 to 20000.

Change-Id: I0c8900e574c91c4c02bbb49416da6df764aab030
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>