firmware: include drivers/firmware/Kconfig unconditionally

Compile-testing drivers that require access to a firmware layer
fails when that firmware symbol is unavailable. This happened
twice this week:

 - My proposed to change to rework the QCOM_SCM firmware symbol
   broke on ppc64 and others.

 - The cs_dsp firmware patch added device specific firmware loader
   into drivers/firmware, which broke on the same set of
   architectures.

We should probably do the same thing for other subsystems as well,
but fix this one first as this is a dependency for other patches
getting merged.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Charles Keepax <ckeepax@opensource.cirrus.com>
Cc: Simon Trimmer <simont@opensource.cirrus.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>