PM / devfreq: make memlat and bwmon governors immutable

The devfreq devices that support the mem_latency, compute
or bw_hwmon governors are tightly coupled with hardware
monitors and are not expected to change governors at
runtime in normal operation. Thus, make these governors
immutable and have them start with min_freq equal to
max_freq to optimize performance during boot. Also make
the initial polling interval 500 ms to reduce polling
activity during boot.

Change-Id: Ida67f404155dce01facdfe999d8e71d9e1ac317c
Signed-off-by: Amir Vajid <avajid@codeaurora.org>