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: Ajay Dudani <adudani@google.com>