firmware_class: make firmware caching configurable

Because firmware caching generates uevent messages that are sent over a
netlink socket, it can prevent suspend on many platforms.  It's also not
always useful, so make it a configurable option.

bug 32180327.

Change-Id: I1250512b27edb56caa78d536e5ccf1fb669476ad
Signed-off-by: Tim Murray <timmurray@google.com>
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.4


Git-commit: e3198d51be98ec5606e2a2c5112a8a974d354fbc
[vnkgutta@codeaurora.org: Resolved trivial merge conflicts]
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>