greybus: firmware.h: rename TFTF firmware filenames

Replace prefix "ara_" with prefix "gmp_" for TFTF firmware filenames.

Testing Done: compiled and flashed EVT2 (bootimage) and tested that
              "gmp_" prefixed tftf files are expected. Pushed one from
	      FDK and checked that it's properly loaded.

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