greybus: move all firmware prefix strings to one place

The firmware prefix is hard-coded in a few different places.  Put it all
in one handy #define, for when/if we ever decide to change it in the
future...

Testing: 'strings gb-firmware.ko gb-bootrom.ko | grep ara_' produced the
	 same output before and after this patch.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Sandeep Patil <sspatil@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>