firmware_class: Provide infrastructure to make fw caching optional

Some low memory systems with complex peripherals cannot
afford to have the relatively large firmware images taking
up valuable memory during suspend and resume. Change the
internal implementation of firmware_class to disallow
caching based on a configurable option. In the near future,
variants of request_firmware will take advantage of this
configurability.

Change-Id: I44be7ce3b308b642fb018086def99fcb800a1109
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>