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

drivers/firmware/broadcom/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config BCM47XX_NVRAM
 	bool "Broadcom NVRAM driver"
-	depends on BCM47XX || ARCH_BCM_5301X
+	depends on BCM47XX || ARCH_BCM_5301X || COMPILE_TEST
 	help
 	  Broadcom home routers contain flash partition called "nvram" with all
 	  important hardware configuration as well as some minor user setup.