firmware: bcm47xx_nvram: Allow COMPILE_TEST

Allow building building the BCM47xx NVRAM and SPROM drivers using
COMPILE_TEST.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@linux-mips.org
Cc: joe@perches.com
Cc: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org